xfstests: rename USE_BIG_LOOPFS to be more generic

xfstests: rename USE_BIG_LOOPFS to be more generic

USE_BIG_LOOPFS is really misnamed - it can be used on real devices just as
easily as loop devices. It really means we are testing a large scratch device
and that we should enable the special filesystem filling and checking options
that enable xfstests to be run sanely on large XFS filesystems.

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