Make pmap_allocate_asid() mpsafe. Since between checking the ASID
table and setting the bits atomically another core can select the
same ASID as we did it currently would not be safe to run it without
the kernel lock. This replaces the atomic_setbits_int(9) call with
atomic_cas_uint(9) where we can check that the table entry has not
been changed since we evaluted it. Also modify pmap_free_asid() to
use the same concept.