Fix zfs_open() to skip zil_async_to_sync() for the snapshot

Fix zfs_open() to skip zil_async_to_sync() for the snapshot, as it won't
have any transactions. zfsvfs->z_log is NULL for the snapshot.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Jitendra Patidar <jitendra.patidar@nutanix.com>
Closes openzfs#18091

Author: jsai20

module/os/freebsd/zfs/zfs_vnops_os.c

@@ -242,8 +242,9 @@ zfs_open(vnode_t **vpp, int flag, cred_t *cr)
 	 * Keep a count of the synchronous opens in the znode.  On first
 	 * synchronous open we must convert all previous async transactions
 	 * into sync to keep correct ordering.
+	 * Skip it for snapshot, as it won't have any transactions.
 	 */
-	if (flag & O_SYNC) {
+	if (!zfsvfs->z_issnap && (flag & O_SYNC)) {
 		if (atomic_inc_32_nv(&zp->z_sync_cnt) == 1)
 			zil_async_to_sync(zfsvfs->z_log, zp->z_id);
 	}
@@ -264,7 +265,7 @@ zfs_close(vnode_t *vp, int flag, int count, offset_t offset, cred_t *cr)
 		return (error);
 
 	/* Decrement the synchronous opens in the znode */
-	if ((flag & O_SYNC) && (count == 1))
+	if (!zfsvfs->z_issnap && (flag & O_SYNC) && (count == 1))
 		atomic_dec_32(&zp->z_sync_cnt);
 
 	zfs_exit(zfsvfs, FTAG);

module/os/linux/zfs/zfs_vnops_os.c

@@ -200,8 +200,9 @@ zfs_open(struct inode *ip, int mode, int flag, cred_t *cr)
 	 * Keep a count of the synchronous opens in the znode.  On first
 	 * synchronous open we must convert all previous async transactions
 	 * into sync to keep correct ordering.
+	 * Skip it for snapshot, as it won't have any transactions.
 	 */
-	if (flag & O_SYNC) {
+	if (!zfsvfs->z_issnap && (flag & O_SYNC)) {
 		if (atomic_inc_32_nv(&zp->z_sync_cnt) == 1)
 			zil_async_to_sync(zfsvfs->z_log, zp->z_id);
 	}
@@ -222,7 +223,7 @@ zfs_close(struct inode *ip, int flag, cred_t *cr)
 		return (error);
 
 	/* Decrement the synchronous opens in the znode */
-	if (flag & O_SYNC)
+	if (!zfsvfs->z_issnap && (flag & O_SYNC))
 		atomic_dec_32(&zp->z_sync_cnt);
 
 	zfs_exit(zfsvfs, FTAG);