dmu_objset: replace dnode_hash impl with cityhash4

As mentioned in PR #16131, replacing CRC-based hash with cityhash4
could slightly improve the performance by eliminating memory access.
Replacing algorightm is safe since the hash result is not persisted.

See: #16131

Signed-off-by: Shengqi Chen <[email protected]>

Author: Harry-Chen

module/zfs/dmu_objset.c

@@ -66,6 +66,7 @@
 #include "zfs_namecheck.h"
 #include <sys/vdev_impl.h>
 #include <sys/arc.h>
+#include <cityhash.h>
 
 /*
  * Needed to close a window in dnode_move() that allows the objset to be freed
@@ -390,27 +391,12 @@ dmu_objset_byteswap(void *buf, size_t size)
 }
 
 /*
- * The hash is a CRC-based hash of the objset_t pointer and the object number.
+ * Runs cityhash4 on the objset_t pointer and the object number.
  */
 static uint64_t
 dnode_hash(const objset_t *os, uint64_t obj)
 {
-	uintptr_t osv = (uintptr_t)os;
-	uint64_t crc = -1ULL;
-
-	ASSERT(zfs_crc64_table[128] == ZFS_CRC64_POLY);
-	/*
-	 * The lower 11 bits of the pointer don't have much entropy, because
-	 * the objset_t is more than 1KB long and so likely aligned to 2KB.
-	 */
-	crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (osv >> 11)) & 0xFF];
-	crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (obj >> 0)) & 0xFF];
-	crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (obj >> 8)) & 0xFF];
-	crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (obj >> 16)) & 0xFF];
-
-	crc ^= (osv>>14) ^ (obj>>24);
-
-	return (crc);
+	return cityhash4((uint64_t)os, obj, 0, 0);
 }
 
 static unsigned int