generic/530: only use xfs-specific mkfs options when testing on xfs

generic/530: only use xfs-specific mkfs options when testing on xfs

This fixes a regression introduced by commit 000813899afb ("fstests:
scale some tests for high CPU count sanity") where this test
unconditionally tried to use the mkfs option "-l size=256m" which
would break when testing any file sytem other than xfs.

Fix this the same way commit 000813899afb dealt with this for
generic/531; so this was just an oversight.

Fixes: 000813899afb ("fstests: scale some tests for high CPU count sanity")
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>