generic: use xfs_io falloc, not fallocate

generic: use xfs_io falloc, not fallocate

generic/315 fails messily if the fallocate command isn't present.

generic/299 also uses "fallocate" and "truncate" binaries which may
not be present.

Switch both to use xfs_io, and we already have the _require for
that, because it's what every other test uses...

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>