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>