mkfs: use cvtnum from libfrog

mkfs: use cvtnum from libfrog

Move the checks for zero block/sector size to the libfrog code
and return -1LL as an invalid value instead. Catch the invalid
value in mkfs and error out there instead of inside cvtnum.

Also rename the libfrog block/sector size variables so they don't
shadow the mkfs global variables of the same name and mark the
string being passed in as a const.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>