mkfs: log stripe unit fails to influence default log size

mkfs: log stripe unit fails to influence default log size

This fails on a 4GB, 4k physical sector size device:

# mkfs.xfs -f -l version=2,su=256k /dev/ram1
log size 2560 blocks too small, minimum size is 3264 blocks
....

The combination of 4k sectors and a log stripe unit increase the
minimum size of the log.  We should be automatically calculating an
appropriate, valid log size when the user does not specify it.

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