Further cleanup after r285607.

Remove useless release semantic for some stores to it_need. For
stores where the release is needed, add a comment explaining why.

Fence after the atomic_cmpset() op on the it_need should be acquire
only, release is not needed (see above). The combination of
atomic_cmpset() + fence_acq() is better expressed there as
atomic_cmpset_acq().

Use atomic_cmpset() for swi' ih_need read and clear.

Discussed with: alc, bde
Reviewed by: bde
Comments wording provided by: bde
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks