xfs: move lru refs to the btree ops structure

Move the btree buffer LRU refcount to the btree ops structure so that we
can eliminate the last bc_btnum switch in the generic btree code.  We're
about to create repair-specific btree types, and we don't want that
stuff cluttering up libxfs.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>