fs: don't open-code mnt_hold_writers()

Remove sb_prepare_remount_readonly()'s open-coded mnt_hold_writers()
implementation with the real helper we introduced in commit fbdc2f6c
("fs: split out functions to hold writers").

Link: https://lore.kernel.org/r/20220203131411.3093040-7-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

@@ -563,12 +563,9 @@ int sb_prepare_remount_readonly(struct super_block *sb)
 	lock_mount_hash();
 	list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
 		if (!(mnt->mnt.mnt_flags & MNT_READONLY)) {
-			mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
-			smp_mb();
-			if (mnt_get_writers(mnt) > 0) {
-				err = -EBUSY;
+			err = mnt_hold_writers(mnt);
+			if (err)
 				break;
-			}
 		}
 	}
 	if (!err && atomic_long_read(&sb->s_remove_count))