btrfs: log new dentries when logging parent dir of a conflicting inode

In the Linux kernel, the following vulnerability has been resolved:

btrfs: log new dentries when logging parent dir of a conflicting inode

If we log the parent directory of a conflicting inode, we are not logging
the new dentries of the directory, so when we finish we have the parent
directory's inode marked as logged but we did not log its new dentries.
As a consequence if the parent directory is explicitly fsynced later and
it does not have any new changes since we logged it, the fsync is a no-op
and after a power failure the new dentries are missing.

Example scenario:

$ mkdir foo

$ sync

$rmdir foo

$ mkdir dir1
$ mkdir dir2

# A file with the same name and parent as the directory we just deleted
# and was persisted in a past transaction. So the deleted directory's
# inode is a conflicting inode of this new file's inode.
$ touch foo

$ ln foo dir2/link

# The fsync on dir2 will log the parent directory (".") because the
# conflicting inode (deleted directory) does not exists anymore, but it
# it does not log its new dentries (dir1).
$ xfs_io -c "fsync" dir2

# This fsync on the parent directory is no-op, since the previous fsync
# logged it (but without logging its new dentries).
$ xfs_io -c "fsync" .

<power failure>

# After log replay dir1 is missing.

Fix this by ensuring we log new dir dentries whenever we log the parent
directory of a no longer existing conflicting inode.

A test case for fstests will follow soon.