Btrfs: fix listxattrs not listing all xattrs packed in the same item

In the listxattrs handler, we were not listing all the xattrs that are
packed in the same btree item, which happens when multiple xattrs have
a name that when crc32c hashed produce the same checksum value.

Fix this by processing them all.

The following test case for xfstests reproduces the issue:

  seq=`basename $0`
  seqres=$RESULT_DIR/$seq
  echo "QA output created by $seq"
  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
  . ./common/attr

  # real QA test starts here
  _supported_fs generic
  _supported_os Linux
  _require_scratch
  _require_attrs

  rm -f $seqres.full

  _scratch_mkfs >>$seqres.full 2>&1
  _scratch_mount

  # Create our test file with a few xattrs. The first 3 xattrs have a name
  # that when given as input to a crc32c function result in the same checksum.
  # This made btrfs list only one of the xattrs through listxattrs system call
  # (because it packs xattrs with the same name checksum into the same btree
  # item).
  touch $SCRATCH_MNT/testfile
  $SETFATTR_PROG -n user.foobar -v 123 $SCRATCH_MNT/testfile
  $SETFATTR_PROG -n user.WvG1c1Td -v qwerty $SCRATCH_MNT/testfile
  $SETFATTR_PROG -n user.J3__T_Km3dVsW_ -v hello $SCRATCH_MNT/testfile
  $SETFATTR_PROG -n user.something -v pizza $SCRATCH_MNT/testfile
  $SETFATTR_PROG -n user.ping -v pong $SCRATCH_MNT/testfile

  # Now call getfattr with --dump, which calls the listxattrs system call.
  # It should list all the xattrs we have set before.
  $GETFATTR_PROG --absolute-names --dump $SCRATCH_MNT/testfile | _filter_scratch

  status=0
  exit

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>