vm_map: Simplify a call to vm_object_allocate_anon()

vm_object_allocate_anon() automatically sets "charge" to 0 if no cred
reference is provided, so the caller doesn't need any conditional logic.

No functional change intended.

Reviewed by: alc, kib
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 70b2996120a7f96527ce5c676f938886e8aa292d)