No functional change intended.
Suggested by: alc
Despite the long name, it's still two characters shorter than the original code. :-)
This could be applied at vm_page.c:787, if it were visible there.
If could be used here, if the fields of vm_phys_fictitious_seg were reordered.
I made the function externally visible.
I think I'd prefer to leave this case as it is. If there were more instances of this pattern, I'd be more inclined to take the suggestion.
Incorporate one of Doug's suggestions.