[XFS] add generic btree types

Add generic union types for btree pointers, keys and records. The generic
btree pointer contains either a 32 and 64bit big endian scalar for short
and long form btrees, and the key and record contain the relevant type for
each possible btree.

Split out from a bigger patch from Dave Chinner and simplified a little
further.

SGI-PV: 985583

SGI-Modid: xfs-linux-melb:xfs-kern:32178a

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