Fix NULL pointer when O_SYNC read in snapshot

When doing read on a file open with O_SYNC, it will trigger zil_commit.
However for snapshot, there's no zil, so we shouldn't be doing that.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes #6478
Closes #6494