mkfs: don't warn about log sunit size if it was auto-discovered

mkfs: don't warn about log sunit size if it was auto-discovered

Today, users doing a bare mkfs on storage with a large default
stripe size may be surprised to get this warning:

 log stripe unit (%d bytes) is too large (maximum is 256KiB
 log stripe unit adjusted to 32KiB

through no fault of their own.  The fallback is appropriate
and harmless, and there's no need to warn about this in the
defaults case.

However, we keep the warning if a large log stripe unit was
specified by the user on the commandline.

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