[XFS] kill di_mode checks after xfs_iget

Unless XFS_IGET_CREATE is passed xfs_iget will return ENOENT if it
encounters an inode with di_mode == 0. Remove the duplicated checks in the
callers.

(the log recovery case is not touched for now)

SGI-PV: 976035
SGI-Modid: xfs-linux-melb:xfs-kern:30898a

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