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 openzfs#6478 
Closes openzfs#6494

module/zfs/zfs_vnops.c

@@ -482,8 +482,10 @@ zfs_read(struct inode *ip, uio_t *uio, int ioflag, cred_t *cr)
 
 	/*
 	 * If we're in FRSYNC mode, sync out this znode before reading it.
+	 * Only do this for non-snapshots.
 	 */
-	if (ioflag & FRSYNC || zfsvfs->z_os->os_sync == ZFS_SYNC_ALWAYS)
+	if (zfsvfs->z_log &&
+	    (ioflag & FRSYNC || zfsvfs->z_os->os_sync == ZFS_SYNC_ALWAYS))
 		zil_commit(zfsvfs->z_log, zp->z_id);
 
 	/*