xfs: don't allocate scrub contexts on the stack

Convert the on-stack scrub context, btree scrub context, and da btree
scrub context into a heap allocation so that we reduce stack usage and
gain the ability to handle tall btrees without issue.

Specifically, this saves us ~208 bytes for the dabtree scrub, ~464 bytes
for the btree scrub, and ~200 bytes for the main scrub context.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Chandan Babu R <chandan.babu@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>