[XFS] implement IHOLD/IRELE directly

[XFS] implement IHOLD/IRELE directly

Now that all direct calls to VN_HOLD/VN_RELE are gone we can implement
IHOLD/IRELE directly.

For the IHOLD case also replace igrab with a direct increment of i_count
because we are guaranteed to already have a live and referenced inode by
the VFS. Also remove the vn_hold statistic because it's been rather
meaningless for some time with most references done by other callers.

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31764a

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