xfstests: fix seek_sanity_test for fs w/o fallocate

xfstests: fix seek_sanity_test for fs w/o fallocate

currently the seek_sanity_test (generic/285) fails on ext3
or ext2 due to fallocate() failures.  Just ignore that test
if the fs doesn't support fallocate.

Note: this patch was originally submitted by Zheng Liu.
http://oss.sgi.com/archives/xfs/2013-05/msg00534.html

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Zheng Liu <gnehzuil.liu@gmail.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>