vm: Move the "vm_wait in early boot" assertion to the proper place

The assertion was added in commit 1771e987ca6a. After that, vm_wait()
and friends were refactored such that the actual sleep happens
elsewhere. Now the assertion condition is not checked when
vm_wait_doms() is called directly, and it is checked even if we are not
going to sleep (because vm_page_count_min_set(wdoms) is false).

Reviewed by: alc, kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34909