vm: Fix address hints of 0 with MAP_32BIT

Also, rename min_addr to default_addr, which better reflects what it
represents. The min_addr is not a minimum address in the same way that
max_addr is actually a maximum address that can be allocated. For
example, a non-zero hint can be less than min_addr and be allocated.

Reported by: dchagin
Reviewed by: dchagin, kib, markj
Fixes: d8e6f4946cec0 "vm: Fix anonymous memory clustering under ASLR"
Differential Revision: https://reviews.freebsd.org/D41397