generic/204: correct log size for XFS

generic/204: correct log size for XFS

generic/204 fails on device with Advanced Format of 4096 bytes per
physical sector and when partition starts at the 4K boundary/./In
this case filesystem sector/block size will be of 4096 bytes size
and _scratch_mkfs_sized fails because mkfs reports that 5Mb log size
is not enough to create a filesystem, for example attempt to make
filesystem on such partition:

mkfs.xfs -f -bsize=4096 -l size=5m -d size=109051904 /dev/sdb2"

results to:

"log size 1280 blocks too small, minimum size is 1605 blocks"

and generic/204 fails with ENOSPC before it has finished creating
the necessary files. Log size of 7MB is enough for this test to pass.

Signed-off-by: Alexander Tsvetkov <alexander.tsvetkov@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>