vm_pageout: Remove a volatile qualifier from some vm_domain members

These are always accessed using atomic(9) intrinsics, so do not need the
qualifier. No functional change intended.

Reviewed by: alc, kib
MFC after: 2 weeks
Sponsored by: Modirum MDPay
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D51322