[XFS] Invalidate dentry in unlink/rmdir if in case-insensitive mode

The vfs_unlink/d_delete functionality in the Linux VFS make the
dentry negative if it is the only inode being referenced. Case-insensitive
mode doesn't work with negative dentries, so if using CI-mode, invalidate
the dentry on unlink/rmdir.

SGI-PV: 983102
SGI-Modid: xfs-linux-melb:xfs-kern:31308a

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