Add a return value to vm_page_remove().

[markj]

Summary

This is a small piece of D20486. In that revision we count the object's
reference in the page's ref_count field, and atomically decrement that
reference and check for others.

All callers except for vm_page_free_prep() must provide a page contained
in an object, so move the m->object != NULL check out of
vm_page_remove().

[dougm]

sys/vm/vm_page.c

1485

Before, object could have been NULL, evidently. Here, you depend on it being not NULL. Can you assert that somewhere?

[markj]

sys/vm/vm_page.c

1485

In general we don't add assertions for conditions which will unconditionally crash the kernel anyway. In particular, the VM_OBJECT_ASSERT_WLOCKED() above effectively provides the same assertion.