Avoid unnecessary zil_commit on rm.

1. Issue zil_commit only if we're actually updating something --
   there's no need to commit if we're unlinking the file or if
   there's no atime update being applied.

2. Issue zil_commit only if the zfs has sync=always set -- for
   sync=standard there's no need for us to commit anything here since
   no application asked for an explicit sync.

Speeds up untarring base.tgz on top of itself by a factor of about
2x, and speeds up rm by a factor of about 10x, on my system with an
SSD SLOG over SATA.  Histogram of unlink, rmdir, and rename timing
shows dramatic reduction in latency for most samples.

(To be fair, this was not an improvement over zfs; issuing the
unnecessary zil_commit was a self-inflicted performance wound.)


(riastradh)