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

[markj]

Summary

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).

[alc]

sys/vm/vm_page.c

3243

As an aside, this variable seems pointless. Each of the 3 cases could end in a return statement.