cifs: Fix kmap_local_page() unmapping

kmap_local_page() requires kunmap_local() to unmap the mapping.  In
addition memcpy_page() is provided to perform this common memcpy
pattern.

Replace the kmap_local_page() and broken kunmap() with memcpy_page()

Fixes: d406d267 ("cifs: skip alloc when request has no pages")
Reviewed-by: Paulo Alcantara <pc@cjr.nz>
Reviewed-by: "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Cc: linux-cifs@vger.kernel.org
Cc: samba-technical@lists.samba.org
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Steve French <stfrench@microsoft.com>