mkfs: pass switch case value directly into getnum/getstr

mkfs: pass switch case value directly into getnum/getstr

Parsing did this sort of thing:

	case D_AGCOUNT:
		cli->agcount = getnum(value, opts, D_AGCOUNT);

which was just begging for a cut and paste error between the
case value and the enum passed into getnum/getstr.  Pass
"subopt" instead so that it is always consistent with the case.

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>