xfs_repair: adjust btree bulkloading slack computations to match online repair

xfs_repair: adjust btree bulkloading slack computations to match online repair

Adjust the lowspace threshold in the new btree block slack computation
code to match online repair, which uses a straight 10% instead of magic
shifting to approximate that without division.  Repairs aren't that
frequent in the kernel; and userspace can always do u64 division.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>