sched: sched_switch(): Factorize sleepqueue flags

Avoid duplicating common flags for the preempted and non-preempted
cases, making it clear that they are the same without resorting to
formatting.

No functional change.

Approved by: markj (mentor)
MFC after: 3 days
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 6a3c02bc52892eb09c3b766562b8ddb452c393a9)

Approved by: emaste (mentor)