Add missing zfs_refcount_destroy() in key_mapping_rele()

Otherwise when running with reference_tracking_enable=TRUE mounting
and unmounting an encrypted dataset panics with:

Call Trace:
 dump_stack+0x66/0x90
 slab_err+0xcd/0xf2
 ? __kmalloc+0x174/0x260
 ? __kmem_cache_shutdown+0x158/0x240
 __kmem_cache_shutdown.cold+0x1d/0x115
 shutdown_cache+0x11/0x140
 kmem_cache_destroy+0x210/0x230
 spl_kmem_cache_destroy+0x122/0x3e0 [spl]
 zfs_refcount_fini+0x11/0x20 [zfs]
 spa_fini+0x4b/0x120 [zfs]
 zfs_kmod_fini+0x6b/0xa0 [zfs]
 _fini+0xa/0x68c [zfs]
 __x64_sys_delete_module+0x19c/0x2b0
 do_syscall_64+0x5b/0x1a0
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reviewed-By: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-By: Tom Caputi <tcaputi@datto.com>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes openzfs#10246 
(cherry picked from commit fa25460)

module/zfs/dsl_crypt.c

@@ -984,6 +984,7 @@ key_mapping_rele(spa_t *spa, dsl_key_mapping_t *km, void *tag)
 	rw_exit(&spa->spa_keystore.sk_km_lock);
 
 	spa_keystore_dsl_key_rele(spa, km->km_key, km);
+	zfs_refcount_destroy(&km->km_refcnt);
 	kmem_free(km, sizeof (dsl_key_mapping_t));
 }