overlay: create directory over deleted whiteout

There's a bug in the overlayfs implementation starting from the very
first merged version that may cause an Oops of various forms if a
directory is created over a whiteout dentry, but the actual whiteout
on the upper layer was removed to the directory creation.

Reported by: kaixuxia <xiakaixu1987@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

tests/overlay/063

@@ -0,0 +1,64 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2018 Red Hat Inc.  All Rights Reserved.
+#
+# FS QA Test 063
+#
+# Create dir over cached negative dentry, but whiteout removed from upper
+#
+# The following kernel commit fixed the kernel crash: 5e1275808630 ("ovl: check
+# whiteout in ovl_create_over_whiteout()")
+#
+seq=`basename $0`
+seqres=$RESULT_DIR/$seq
+echo "QA output created by $seq"
+
+here=`pwd`
+tmp=/tmp/$$
+status=1	# failure is the default!
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+_cleanup()
+{
+	cd /
+	rm -f $tmp.*
+}
+
+# get standard environment, filters and checks
+. ./common/rc
+. ./common/filter
+
+# remove previous $seqres.full before test
+rm -f $seqres.full
+
+# real QA test starts here
+_supported_fs overlay
+_supported_os Linux
+_require_scratch
+
+# Remove all files from previous tests
+_scratch_mkfs
+
+# Create test file
+lowerdir=${OVL_BASE_SCRATCH_MNT}/${OVL_LOWER}
+upperdir=${OVL_BASE_SCRATCH_MNT}/${OVL_UPPER}
+mkdir -p $lowerdir
+touch ${lowerdir}/file
+
+_scratch_mount
+
+# Create whiteout and populate dcache with negative dentry
+rm ${SCRATCH_MNT}/file
+ls -l ${SCRATCH_MNT}/file > /dev/null 2>&1
+
+# Remove whiteout and try to create dir over negative dentry
+rm ${upperdir}/file
+mkdir ${SCRATCH_MNT}/file > /dev/null 2>&1
+
+# unmount overlayfs
+$UMOUNT_PROG $SCRATCH_MNT
+
+echo "Silence is golden"
+# success, all done
+status=0
+exit

tests/overlay/063.out

@@ -0,0 +1,2 @@
+QA output created by 063
+Silence is golden

tests/overlay/group

@@ -65,3 +65,4 @@
 060 auto quick metacopy
 061 auto quick copyup
 062 auto quick exportfs
+063 auto quick whiteout