vm_page_advise(): reformat conditional statements

[mhorne]

Summary

Use a switch, for better enumeration of the possible values. This is
non-functional change, but IMO it makes the logic a little easier to
follow.

[kib]

sys/vm/vm_page.c

4453

Suppose that advise is MADV_WILLNEED. Before the patch, the page is dirtied if pmap thinks it is modified.

[alc]

sys/vm/vm_page.c

4453

If advise is MADV_WILLNEED, we don't get to this point. We return on line 4450.

[markj]

sys/vm/vm_page.c

4459

This change simplifies the control flow, but I don't know that the code is really clearer. The handling the dirty bits for MADV_FREE/DONTNEED above is connected to the rest of the function below, but that's not obvious. I think this would be the easiest to follow:

if (advice != MADV_DONTNEED && advice != MADV_FREE) {
    if (advice == MADV_WILLNEED)
        vm_page_activate(m);
    return;
}

if (advice == MADV_FREE)
    vm_page_undirty(m);
else if (m->dirty == 0 && pmap_is_modified(m))
    vm_page_dirty(m);
...