[XFS] Fix returning case-preserved name with CI node form directories

[XFS] Fix returning case-preserved name with CI node form directories

xfs_dir2_node_lookup() calls xfs_da_node_lookup_int() which iterates
through leaf blocks containing the matching hash value for the name being
looked up. Inside xfs_da_node_lookup_int(), it calls the
xfs_dir2_leafn_lookup_for_entry() for each leaf block.
xfs_dir2_leafn_lookup_for_entry() iterates through each matching
hash/offset pair doing a name comparison to find the matching dirent.

For CI mode, the state->extrablk retains the details of the block that has
the CI match so xfs_dir2_node_lookup() can return the case-preserved name.

The original implementation didn't retain the xfs_da_buf_t properly, so
the lookup was returning a bogus name to be stored in the dentry.

In the case of unlink, the bad name was passed and in debug mode, ASSERTed
when it can't find the entry.

SGI-PV: 983284

SGI-Modid: xfs-linux-melb:xfs-kern:31337a

Signed-off-by: Barry Naujok <bnaujok@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>