cifs: add cifs_revalidate_file

...to allow updating inode attributes on an existing inode by
filehandle. Change mmap and llseek codepaths to use that
instead of cifs_revalidate_dentry since they have a filehandle
readily available.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

diff --cc fs/cifs/cifsfs.c
Simple merge

diff --cc fs/cifs/cifsfs.h
Simple merge

diff --cc fs/cifs/cifsproto.h
Simple merge

diff --cc fs/cifs/file.c
Simple merge

diff --cc fs/cifs/inode.c
Simple merge