mkfs: round log size down if rounding log start up causes overflow

mkfs: round log size down if rounding log start up causes overflow

If rounding the log start up to the next stripe unit would cause the log
to overrun the end of the AG, round the log size down by a stripe unit.
We already ensured that logblocks was small enough to fit inside the AG,
so the minor adjustment should suffice.

This can be reproduced with:
mkfs.xfs -dsu=44k,sw=1,size=300m,file,name=fsfile -m rmapbt=0
and:
mkfs.xfs -dsu=48k,sw=1,size=512m,file,name=fsfile -m rmapbt=0

Reported-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>