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
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35781