generic/25[02]: increase filesystem size

generic/25[02]: increase filesystem size

On reflink+rmapbt XFs filesystems there isn't enough free space to
run this test on the 64MB filesystem image created. It notruns with
a curious error message - needs at least 0GB free:

generic/250             [10:01:57] [10:01:58] [not run]
        generic/250 -- This test requires at least 0GB free on /mnt/scratch to run

Fix this by increasing the size of the base filesystem image.

Signed-Off-By: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>