mkfs: fix missing validation of -l size against maximum internal log size

mkfs: fix missing validation of -l size against maximum internal log size

If a sysadmin specifies a log size explicitly, we don't actually check
that against the maximum internal log size that we compute for the
default log size computation.  We're going to add more validation soon,
so refactor the max internal log blocks into a common variable and
add a check.

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