Do not lock vm map in swapout_procs().

Neither swapout_procs() nor swapout() access the map. Since the
process' vmspace is referenced only to obtain the pointer to the
vm_map, the reference is not needed as well.

Reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D13681