poll/select: change selfd_zone into a malloc type

On a sample box vmstat -z shows:

ITEM SIZE LIMIT USED FREE REQ
64: 64, 0, 1043784, 4367538,3698187229
selfd: 64, 0, 1520, 13726,182729008

But at the same time:
vm.uma.selfd.keg.domain.1.pages: 121
vm.uma.selfd.keg.domain.0.pages: 121

Thus 242 pages got pulled even though the malloc zone would likely accomodate
the load without using extra memory.