xfs: remove trivial bnobt/inobt scrub helpers

Christoph Hellwig complained about awkward code in the next two repair
patches such as:

	sc->sm->sm_type = XFS_SCRUB_TYPE_BNOBT;
	error = xchk_bnobt(sc);

This is a little silly, so let's export the xchk_{,i}allocbt functions
to the dispatch table in scrub.c directly and get rid of the helpers.
Originally I had planned each btree gets its own separate entry point,
but since repair doesn't work that way, it no longer makes sense to
complicate the call chain that way.

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