xfstests: use minimum log size in xfs/016

xfstests: use minimum log size in xfs/016

Test xfs/016 fails to run due to invalid mkfs options. The log size
is reported as too small according to the minimum log size
calculation:

log size 512 blocks too small, minimum size is 853 blocks

Update log_size to the currently specified minimum.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>