Revert "kern/vfs_vnops.c: generalize the lock primitive for file foffset"

Temporarily back this out to fix the tree while I try 16.0 snapshot
builds. It will come back once D52626 lands.

This reverts commit 94a0f9f63e164e4f3791bc7d105d6f267b4d47bc.

Discussed with: kib