ovl: inode reference leak in ovl_is_inuse true case.

to #29273482

When "ovl_is_inuse" true case, trap inode reference not put.  plus adding
the comment explaining sequence of ovl_is_inuse after ovl_setup_trap.

Fixes: 0be0bfd2de9d ("ovl: fix regression caused by overlapping layers detection")
Cc: <stable@vger.kernel.org> # v4.19+
Reviewed-by: NAmir Goldstein <amir73il@gmail.com>
Signed-off-by: Nyoungjun <her0gyugyu@gmail.com>
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>
Link: https://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs.git/commit/?h=overlayfs-next&id=24f14009b8f1754ec2ae4c168940c01259b0f88aSigned-off-by: NJeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: NJoseph Qi <joseph.qi@linux.alibaba.com>

fs/overlayfs/super.c

@@ -1322,14 +1322,23 @@ static int ovl_get_lower_layers(struct super_block *sb, struct ovl_fs *ofs,
 		if (err < 0)
 			goto out;
 
+		/*
+		 * Check if lower root conflicts with this overlay layers before
+		 * checking if it is in-use as upperdir/workdir of "another"
+		 * mount, because we do not bother to check in ovl_is_inuse() if
+		 * the upperdir/workdir is in fact in-use by our
+		 * upperdir/workdir.
+		 */
 		err = ovl_setup_trap(sb, stack[i].dentry, &trap, "lowerdir");
 		if (err)
 			goto out;
 
 		if (ovl_is_inuse(stack[i].dentry)) {
 			err = ovl_report_in_use(ofs, "lowerdir");
-			if (err)
+			if (err) {
+				iput(trap);
 				goto out;
+			}
 		}
 
 		mnt = clone_private_mount(&stack[i]);