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
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 6fb7c42d593d13eba8b72f6cdf434a7cb23fa828)