introduced guards for mount_lock

introduced guards for mount_lock

mount_writer: write_seqlock; that's an equivalent of {un,}lock_mount_hash()
mount_locked_reader: read_seqlock_excl; these tend to be open-coded.

No bulk conversions, please - if nothing else, quite a few places take
use mount_writer form when mount_locked_reader is sufficent.  It needs
to be dealt with carefully.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>