dmu_buf_will_clone: change assertion to fix 32-bit compiler warning

Building module/zfs/dbuf.c for 32-bit targets can result in a warning:

In file included from
/usr/src/sys/contrib/openzfs/include/sys/zfs_context.h:97,
                 from /usr/src/sys/contrib/openzfs/module/zfs/dbuf.c:32:
/usr/src/sys/contrib/openzfs/module/zfs/dbuf.c: In function
'dmu_buf_will_clone':
/usr/src/sys/contrib/openzfs/lib/libspl/include/assert.h:116:33: error:
cast from pointer to integer of different size
[-Werror=pointer-to-int-cast]
  116 |         const uint64_t __left = (uint64_t)(LEFT);
  \
      |                                 ^
/usr/src/sys/contrib/openzfs/lib/libspl/include/assert.h:148:25: note:
in expansion of macro 'VERIFY0'
  148 | #define ASSERT0         VERIFY0
      |                         ^~~~~~~
/usr/src/sys/contrib/openzfs/module/zfs/dbuf.c:2704:9: note: in
expansion of macro 'ASSERT0'
 2704 |         ASSERT0(dbuf_find_dirty_eq(db, tx->tx_txg));
      |         ^~~~~~~

This is because dbuf_find_dirty_eq() returns a pointer, which if
pointers are 32-bit results in a warning about the cast to uint64_t.

Instead, use the ASSERT3P() macro, with == and NULL as second and third
arguments, which should work regardless of the target's bitness.

Reviewed-by: Kay Pedersen <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Brian Atkinson <[email protected]>
Signed-off-by: Dimitry Andric <[email protected]>
Closes #15224

module/zfs/dbuf.c

@@ -2701,7 +2701,7 @@ dmu_buf_will_clone(dmu_buf_t *db_fake, dmu_tx_t *tx)
 	 */
 	mutex_enter(&db->db_mtx);
 	VERIFY(!dbuf_undirty(db, tx));
-	ASSERT0(dbuf_find_dirty_eq(db, tx->tx_txg));
+	ASSERT3P(dbuf_find_dirty_eq(db, tx->tx_txg), ==, NULL);
 	if (db->db_buf != NULL) {
 		arc_buf_destroy(db->db_buf, db);
 		db->db_buf = NULL;