CIFS: Show backupuid/gid in /proc/mounts

Show  backupuid/backupgid in /proc/mounts for cifs shares mounted with
the backupuid/backupgid feature.

Also consolidate the two separate checks for
pvolume_info->backupuid_specified into a single if condition in
cifs_setup_cifs_sb().

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