amd64: a simplication to pmap_remove_{all,write}

[alc]

Summary

Eliminate some unnecessary unlocking and relocking when we have to retry the operation to avoid deadlock. (All of the other pmap functions that iterate over a PV list already implemented retries without these same unlocking and relocking operations.)

[kib]

sys/amd64/amd64/pmap.c

8509

Why did you left out the spin chunk from there? Because it would require larger change to fcmset to re-read oldpte to avoid loosing PG_M?

[alc]

sys/amd64/amd64/pmap.c

8509

Yes. I'll try doing it separately.