Check vdev map has space before calling ffs,
fix invalid cleanup in failure to create vdev
case.

Open-code the BIT() logic since BIT does not properly
handle 64-bit bitfields and future patches will make
use of larger bitfields.
Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>