Split out flag manipulation from general context manipulation in iflib

To avoid blocking on the context lock in the swi thread and risk potential
deadlocks, this change protects lighter weight updates that only need to
be consistent with each other with their own lock.

Submitted by: Matthew Macy <mmacy@mattmacy.io>
Reviewed by: shurd
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D14967