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>

fs/mount.h

@@ -154,6 +154,11 @@ static inline void get_mnt_ns(struct mnt_namespace *ns)
 
 extern seqlock_t mount_lock;
 
+DEFINE_LOCK_GUARD_0(mount_writer, write_seqlock(&mount_lock),
+		    write_sequnlock(&mount_lock))
+DEFINE_LOCK_GUARD_0(mount_locked_reader, read_seqlock_excl(&mount_lock),
+		    read_sequnlock_excl(&mount_lock))
+
 struct proc_mounts {
 	struct mnt_namespace *ns;
 	struct path root;