At the time opt-in was introduced adding yourself as a writer was esrializing
across the mount point. Nowadays it is fully per-cpu, the only impact being
a small single-threaded hit on top of what's there right now.
Vast majority of the overhead stems from the call to VOP_GETWRITEMOUNT which
has is done regardless.
Should someone want to microoptimize this single-threaded they can coalesce
looking the mount up with adding a write to it.
074ad60a4c