[XFS] mark various functions in xfs_btree.c static

Lots of functionality in xfs_btree.c isn't needed by callers outside of
this file anymore, so mark these functions static.

SGI-PV: 985583

SGI-Modid: xfs-linux-melb:xfs-kern:32209a

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>