[XFS] Remove vn_from_inode()

bhv_vnode_t is just a typedef for struct inode, so there's
no need for a helper to convert between the two.

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31760a

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