xfs: add RENAME_WHITEOUT support

Whiteouts are used by overlayfs -  it has a crazy convention that a
whiteout is a character device inode with a major:minor of 0:0.
Because it's not documented anywhere, here's an example of what
RENAME_WHITEOUT does on ext4:

# echo foo > /mnt/scratch/foo
# echo bar > /mnt/scratch/bar
# ls -l /mnt/scratch
total 24
-rw-r--r-- 1 root root     4 Feb 11 20:22 bar
-rw-r--r-- 1 root root     4 Feb 11 20:22 foo
drwx------ 2 root root 16384 Feb 11 20:18 lost+found
# src/renameat2 -w /mnt/scratch/foo /mnt/scratch/bar
# ls -l /mnt/scratch
total 20
-rw-r--r-- 1 root root     4 Feb 11 20:22 bar
c--------- 1 root root  0, 0 Feb 11 20:23 foo
drwx------ 2 root root 16384 Feb 11 20:18 lost+found
# cat /mnt/scratch/bar
foo
#

In XFS rename terms, the operation that has been done is that source
(foo) has been moved to the target (bar), which is like a nomal
rename operation, but rather than the source being removed, it have
been replaced with a whiteout.

We can't allocate whiteout inodes within the rename transaction due
to allocation being a multi-commit transaction: rename needs to
be a single, atomic commit. Hence we have several options here, form
most efficient to least efficient:

    - use DT_WHT in the target dirent and do no whiteout inode
      allocation.  The main issue with this approach is that we need
      hooks in lookup to create a virtual chardev inode to present
      to userspace and in places where we might need to modify the
      dirent e.g. unlink.  Overlayfs also needs to be taught about
      DT_WHT. Most invasive change, lowest overhead.

    - create a special whiteout inode in the root directory (e.g. a
      ".wino" dirent) and then hardlink every new whiteout to it.
      This means we only need to create a single whiteout inode, and
      rename simply creates a hardlink to it. We can use DT_WHT for
      these, though using DT_CHR means we won't have to modify
      overlayfs, nor anything in userspace. Downside is we have to
      look up the whiteout inode on every operation and create it if
      it doesn't exist.

    - copy ext4: create a special whiteout chardev inode for every
      whiteout.  This is more complex than the above options because
      of the lack of atomicity between inode creation and the rename
      operation, requiring us to create a tmpfile inode and then
      linking it into the directory structure during the rename. At
      least with a tmpfile inode crashes between the create and
      rename doesn't leave unreferenced inodes or directory
      pollution around.

By far the simplest thing to do in the short term is to copy ext4.
While it is the most inefficient way of supporting whiteouts, but as
an initial implementation we can simply reuse existing functions and
add a small amount of extra code the the rename operation.

When we get full whiteout support in the VFS (via the dentry cache)
we can then look to supporting DT_WHT method outlined as the first
method of supporting whiteouts. But until then, we'll stick with
what overlayfs expects us to be: dumb and stupid.

Signed-off-by: Dave Chinner <dchinner@redhat.com>