mkfs: factor writing AG headers

mkfs: factor writing AG headers

There are some slight changes to the way log alignment is calculated
in the change. Instead of using a flag, it checks the log start
block to see if it's different to the first free block in the log
AG, and if it is different then does the aligned setup. This means
we no longer have to care if the log is aligned or not, the code
will do the right thing in all cases.

Signed-Off-By: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>