Restructure swapout tests after vm map locking was removed.

Consolidate the regions covered by the process lock.
Combine similar conditions tests into one, e.g. all process flags can
be test with one logical operation.
Add check for in-exec state, since p_vmspace is dererenced.
Remove labels and goto by explicitly tracking state.
Update comments.

Reviewed by: alc, markj (previous version)
Tested by: pho (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D13693