userns: fix a struct's kernel-doc notation

Use the 'struct' keyword for a struct's kernel-doc notation to avoid a
kernel-doc warning:

kernel/user_namespace.c:232: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * idmap_key struct holds the information necessary to find an idmapping in a

Link: https://lkml.kernel.org/r/20230108021243.16683-1-rdunlap@infradead.org


Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Eric Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>