xfs: rename btree helpers that depends on the block number representation

All these helpers hardcode fsblocks or agblocks and not just the pointer
size.  Rename them so that the names are still fitting when we add the
long format in-memory blocks and adjust the checks when calling them to
check the btree types and not just pointer length.

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>