xen/shbuf: eliminate 17 kernel-doc warnings

Don't use kernel-doc markers ("/**") for comments that are not in
kernel-doc format. This prevents multiple kernel-doc warnings:

xen-front-pgdir-shbuf.c:25: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * This structure represents the structure of a shared page
xen-front-pgdir-shbuf.c:37: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Shared buffer ops which are differently implemented
xen-front-pgdir-shbuf.c:65: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Get granted reference to the very first page of the
xen-front-pgdir-shbuf.c:85: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Map granted references of the shared buffer.
xen-front-pgdir-shbuf.c:106: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Unmap granted references of the shared buffer.
xen-front-pgdir-shbuf.c:127: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Free all the resources of the shared buffer.
xen-front-pgdir-shbuf.c:154: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Get the number of pages the page directory consumes itself.
xen-front-pgdir-shbuf.c:164: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Calculate the number of grant references needed to share the buffer
xen-front-pgdir-shbuf.c:176: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Calculate the number of grant references needed to share the buffer
xen-front-pgdir-shbuf.c:194: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Unmap the buffer previously mapped with grant references
xen-front-pgdir-shbuf.c:242: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Map the buffer with grant references provided by the backend.
xen-front-pgdir-shbuf.c:324: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Fill page directory with grant references to the pages of the
xen-front-pgdir-shbuf.c:354: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Fill page directory with grant references to the pages of the
xen-front-pgdir-shbuf.c:393: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Grant references to the frontend's buffer pages.
xen-front-pgdir-shbuf.c:422: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Grant all the references needed to share the buffer.
xen-front-pgdir-shbuf.c:470: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Allocate all required structures to mange shared buffer.
xen-front-pgdir-shbuf.c:510: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Allocate a new instance of a shared buffer.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Closes: lore.kernel.org/r/202311060203.yQrpPZhm-lkp@intel.com
Acked-by: Juergen Gross <jgross@suse.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Cc: xen-devel@lists.xenproject.org
Link: https://lore.kernel.org/r/20231106055631.21520-1-rdunlap@infradead.org


Signed-off-by: Juergen Gross <jgross@suse.com>