minixfs: fix block limit check

On minix2 and minix3 usually max_size is 7fffffff and the check in
question prohibits creation of last block spanning right before 7fffffff,
due to downward rounding during the division.  Fix it by using
multiplication instead.

[akpm@linux-foundation.org: fix up code layout, use local `sb']
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>