blist: use swblk_t type (defined in sys/blist.h)

blist: use swblk_t type (defined in sys/blist.h)

reduce the diff with DragonFlyBSD by using swblk_t and u_swblk_t types.

while here, move bitmap type (u_swblk_t) to u_int64_t on all archs. it makes the
regress the same on 64 and 32bits archs (and it success on both).

ok mpi@