cifs: pull freeing mountdata/dropping nls/freeing cifs_sb into cifs_umount()

all callers of cifs_umount() proceed to do the same thing; pull it into
cifs_umount() itself.

Acked-by: Pavel Shilovsky <piastryyy@gmail.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>