xfs: compute maximum AG btree height for critical reservation calculation

Compute the actual maximum AG btree height for deciding if a per-AG
block reservation is critically low.  This only affects the sanity check
condition, since we /generally/ will trigger on the 10% threshold.  This
is a long-winded way of saying that we're removing one more usage of
XFS_BTREE_MAXLEVELS.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>