xfstests: fix src/seek_sanity_test -t option

xfstests: fix src/seek_sanity_test -t option

_require_seek_data_hole() does not work because
the -t (test) option of seek_sanity_test is broken,
because of an early check for (argc != 2):

# src/seek_sanity_test -t foo
Usage: src/seek_sanity_test base_file_path

So _require_seek_data_hole() doesn't see the
"Kernel does not support" string it's looking for,
and passes the check.

So rather than _notrun-ing the test, it proceeds to
fail with noisy errors.

Fix that, make a common usage() function, and check for
too many args as well.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Jie Liu <jeff.liu@oracle.com>
Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>