fs: simplify check in mount_setattr_commit()

In order to determine whether we need to call mnt_unhold_writers() in
mount_setattr_commit() we currently do not just check whether
MNT_WRITE_HOLD is set but also if a read-only mount was requested.

However, checking whether MNT_WRITE_HOLD is set is enough. Setting
MNT_WRITE_HOLD requires lock_mount_hash() to be held and it must be
unset before calling unlock_mount_hash(). This guarantees that if we see
MNT_WRITE_HOLD we know that we were the ones who set it earlier. We
don't need to care about why we set it. Plus, leaving this additional
read-only check in makes the code more confusing because it implies that
MNT_WRITE_HOLD could've been set by another thread when it really can't.
Remove it and update the associated comment.

Link: https://lore.kernel.org/r/20220203131411.3093040-6-brauner@kernel.org
Cc: Seth Forshee <seth.forshee@digitalocean.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/namespace.c

@@ -4096,13 +4096,8 @@ static void mount_setattr_commit(struct mount_kattr *kattr,
 			WRITE_ONCE(m->mnt.mnt_flags, flags);
 		}
 
-		/*
-		 * We either set MNT_READONLY above so make it visible
-		 * before ~MNT_WRITE_HOLD or we failed to recursively
-		 * apply mount options.
-		 */
-		if ((kattr->attr_set & MNT_READONLY) &&
-		    (m->mnt.mnt_flags & MNT_WRITE_HOLD))
+		/* If we had to hold writers unblock them. */
+		if (m->mnt.mnt_flags & MNT_WRITE_HOLD)
 			mnt_unhold_writers(m);
 
 		if (!err && kattr->propagation)