overlay: copy up and remove from another namespace

Via unshare, copy up dir from lower dir then remove
them in another unshared namespace.

This would fail before v4.7 kernel with EPERM.

Kernel commit 3fe6e52f
  ovl: override creds with the ones from the superblock mounter
fixed this issue, with this reproducer provided. Original
reproducer was written by commit author:
Antonio Murdaca <amurdaca@redhat.com>

Signed-off-by: Xiong Zhou <xzhou@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

tests/overlay/020

@@ -0,0 +1,70 @@
+#! /bin/bash
+# FS QA Test 020
+#
+# Reproducer from kernel commit 3fe6e52
+#    ovl: override creds with the ones from the superblock mounter
+# Testing ovl copyup/namespace/cred issue.
+#
+#-----------------------------------------------------------------------
+# Copyright (c) 2016 Red Hat Inc. All Rights Reserved.
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it would be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write the Free Software Foundation,
+# Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+#-----------------------------------------------------------------------
+#
+
+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 $seqres.full
+
+# real QA test starts here
+
+# Modify as appropriate.
+_supported_fs overlay
+_supported_os Linux
+_require_scratch
+
+# Remove all files from previous tests
+_scratch_mkfs
+
+lowerdir=$SCRATCH_DEV/$OVERLAY_LOWER_DIR
+mkdir -p $lowerdir/dir
+
+_scratch_mount
+
+unshare -m -p -f -U -r bash -c \
+	"touch $SCRATCH_MNT/dir/{1,2,3}; rm -fR $SCRATCH_MNT/*"
+
+echo "Silence is golden"
+
+# success, all done
+status=0
+exit

tests/overlay/020.out

@@ -0,0 +1,2 @@
+QA output created by 020
+Silence is golden

tests/overlay/group

@@ -22,3 +22,4 @@
 017 auto quick copyup
 018 auto quick copyup
 019 auto stress
+020 auto quick copyup perms