everywhere: misc unnecessary var init/update

These are all cases where we initialise or update a variable, and then
never use it. None of them particularly matter, as the compiler should
optimise them all away during dead store elimination, but some static
analysers complain about them and they are extra work for casual readers
to follow, so worth removing.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #17551

module/zfs/zap.c

@@ -1304,7 +1304,7 @@ zap_increment_int(objset_t *os, uint64_t obj, uint64_t key, int64_t delta,
 int
 fzap_cursor_retrieve(zap_t *zap, zap_cursor_t *zc, zap_attribute_t *za)
 {
-	int err = ENOENT;
+	int err;
 	zap_entry_handle_t zeh;
 	zap_leaf_t *l;
 

module/zfs/zio.c

@@ -4408,7 +4408,7 @@ int
 zio_alloc_zil(spa_t *spa, objset_t *os, uint64_t txg, blkptr_t *new_bp,
     uint64_t size, boolean_t *slog)
 {
-	int error = 1;
+	int error;
 	zio_alloc_list_t io_alloc_list;
 
 	ASSERT(txg > spa_syncing_txg(spa));