After a succession of renames operations of different files and unlinking
one of them, if we fsync one of the renamed files we can end up with a
log that will either fail to replay at mount time or result in a filesystem
that is in an inconsistent state. One example scenario:

  $ mkfs.btrfs -f /dev/sdb
  $ mount /dev/sdb /mnt

  $ mkdir /mnt/testdir
  $ touch /mnt/testdir/fname1
  $ touch /mnt/testdir/fname2

  $ sync

  $ mv /mnt/testdir/fname1 /mnt/testdir/fname3
  $ rm -f /mnt/testdir/fname2
  $ ln /mnt/testdir/fname3 /mnt/testdir/fname2

  $ touch /mnt/testdir/fname1
  $ xfs_io -c "fsync" /mnt/testdir/fname1

  <power failure>

  $ mount /dev/sdb /mnt
  $ umount /mnt
  $ btrfs check /dev/sdb
  [1/7] checking root items
  [2/7] checking extents
  [3/7] checking free space cache
  [4/7] checking fs roots
  root 5 inode 259 errors 2, no orphan item
  ERROR: errors found in fs roots
  Opening filesystem to check...
  Checking filesystem on /dev/sdc
  UUID: 20e4abb8-5a19-4492-8bb4-6084125c2d0d
  found 393216 bytes used, error(s) found
  total csum bytes: 0
  total tree bytes: 131072
  total fs tree bytes: 32768
  total extent tree bytes: 16384
  btree space waste bytes: 122986
  file data blocks allocated: 262144
   referenced 262144

On a kernel without the first patch in this series, titled
"[PATCH] Btrfs: fix fsync after succession of renames of different files",
we get instead an error when mounting the filesystem due to failure of
replaying the log:

  $ mount /dev/sdb /mnt
  mount: mount /dev/sdb on /mnt failed: File exists

Fix this by logging the parent directory of an inode whenever we find an
inode that no longer exists (was unlinked in the current transaction),
during the procedure which finds inodes that have old names that collide
with new names of other inodes.

A test case for fstests follows soon.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>