[XFS] Remove VN_IS* macros and related cruft.

We can just check i_mode / di_mode directly.

SGI-PV: 976035
SGI-Modid: xfs-linux-melb:xfs-kern:30896a

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