xfstests: restrict 259 device sizes to multiples of 512

xfstests: restrict 259 device sizes to multiples of 512

Test 259 tries to make a loop device size which is 1 byte less
than 4T; losetup now warns that this makes little sense, and
the warning breaks the test output:

    +losetup: /mnt/test/259.image: warning: file does not fit into a 512-byte sector the end of the file will be ignored.

The RH QE testcase did originally use loopback, so did
not in effect test anything other than 512-multiple boundaries.

Just drop the non-512-byte-multiple cases, they produce
devices exactly the same size as their 512-byte-multiple
neighbors.

(FWIW, this is a regression test for the bug that
d943b11 mkfs: get size of device properly
fixed.)

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>