xfs: remove the which variable in xchk_iallocbt

The which variable that holds a btree number is passed to two functions
that ignore it and used in a single check that can check the sm_type
as well.  Remove it to unclutter the code.

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