vmm: Simplify the VM_ALLOC_MEMSEG ioctl a bit

We can free the mask earlier, simplifying some error paths. No
functional change intended.

Reviewed by: corvink, jhb, emaste
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D53418