generic/251: skip this test if fstrim geometry detection fails

generic/251: skip this test if fstrim geometry detection fails

There's no reason to wait until fstrim_loop to detect the length
constraints on the FITRIM call since failing to detect any constraints
just leads to _notrun.  Run that stuff from the top level scope so that
we don't start up a bunch of exercisers for no good reason.

Cc: fstests@vger.kernel.org # v2023.10.29
Fixes: 95b0db739400c2 ("generic/251: check min and max length and minlen for FSTRIM")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>