xfs: allow unlinked symlinks and dirs with zero size

For a very very long time, inode inactivation has set the inode size to
zero before unmapping the extents associated with the data fork.
Unfortunately, commit 3c6f46ea changed the inode verifier to
prohibit zero-length symlinks and directories.  If an inode happens to
get logged in this state and the system crashes before freeing the
inode, log recovery will also fail on the broken inode.

Therefore, allow zero-size symlinks and directories as long as the link
count is zero; nobody will be able to open these files by handle so
there isn't any risk of data exposure.

Fixes: 3c6f46ea

 ("xfs: sanity check directory inode di_size")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>