Fix zfs_znode_sa_init() panic by updating ZFS_OBJ_MUTEX()

The time at which a zp's z_zfsvfs is assigned was changed in
upstream here:

illumos/illumos-gate@b5fca8f

Although we had already merged in most of that change, we had
neglected to update the definition of ZFS_OBJ_MUTEX() to the new
two-argument version. Yet, when actually invoking the macro, we
were supplying two arguments.

In an attempt to reconcile the discrepancy, the panic was
introduced by

8881f60

which erroneously reverted the new code where the two-argument
version was used, rather than updating ZFS_OBJ_MUTEX() to the new
two-argument version.

This results in panic because ZFS_OBJ_MUTEX() happens to be used
before zp->z_zfsvfs has been assigned, while the one-argument
version of ZFS_OBJ_MUTEX() expects to be able to retrieve the
zfsvfs from the zp.

include/sys/zfs_znode.h

@@ -310,9 +310,8 @@ typedef struct znode {
  * Macros for dealing with dmu_buf_hold
  */
 #define	ZFS_OBJ_HASH(obj_num)	((obj_num) & (ZFS_OBJ_MTX_SZ - 1))
-
-#define ZFS_OBJ_MUTEX(zp)       \
-        (&zp->z_zfsvfs->z_hold_mtx[ZFS_OBJ_HASH(zp->z_id)])
+#define	ZFS_OBJ_MUTEX(zfsvfs, obj_num)	\
+	(&(zfsvfs)->z_hold_mtx[ZFS_OBJ_HASH(obj_num)])
 #define ZFS_OBJ_HOLD_ENTER(zfsvfs, obj_num) \
         mutex_enter(&zfsvfs->z_hold_mtx[ZFS_OBJ_HASH(obj_num)])
 #define ZFS_OBJ_HELD(zfsvfs, obj_num) \

module/zfs/zfs_znode.c

@@ -624,7 +624,7 @@ zfs_znode_sa_init(zfsvfs_t *zfsvfs, znode_t *zp,
 				    sa_handle_t *sa_hdl)
 {
 	ASSERT(!POINTER_IS_VALID(zp->z_zfsvfs) || (zfsvfs == zp->z_zfsvfs));
-	ASSERT(MUTEX_HELD(ZFS_OBJ_MUTEX(zp)));
+	ASSERT(MUTEX_HELD(ZFS_OBJ_MUTEX(zfsvfs, zp->z_id)));
 
 	mutex_enter(&zp->z_lock);
 
@@ -655,7 +655,7 @@ zfs_znode_sa_init(zfsvfs_t *zfsvfs, znode_t *zp,
 void
 zfs_znode_dmu_fini(znode_t *zp)
 {
-	ASSERT(MUTEX_HELD(ZFS_OBJ_MUTEX(zp)) ||
+	ASSERT(MUTEX_HELD(ZFS_OBJ_MUTEX(zp->z_zfsvfs, zp->z_id)) ||
 	    zp->z_unlinked ||
 	    RW_WRITE_HELD(&zp->z_zfsvfs->z_teardown_inactive_lock));