[XFS] factor xfs_iget_core() into hit and miss cases

There are really two cases in xfs_iget_core(). The first is the cache hit
case, the second is the miss case. They share very little code, and hence
can easily be factored out into separate functions. This makes the code
much easier to understand and subsequently modify.

SGI-PV: 988141

SGI-Modid: xfs-linux-melb:xfs-kern:32317a

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