mmap(2): If we fail with a hint, try again without it.

`Hint' here means nonzero addr, but no MAP_FIXED or MAP_TRYFIXED.

This is suboptimal -- we could teach uvm_mmap to do a fancier search
using the address as a hint.  But this should do for now.

Candidate fix for PR kern/55533.

ok chs@

(riastradh)