Silence static analyzer warnings about spa_sync_props()

Both Coverity and Clang's static analyzer complain about reading an
uninitialized intval if the property is not passed as DATA_TYPE_UINT64
in the nvlist. This is impossible becuase spa_prop_validate() already
checked this, but they are unlikely to be the last static analyzers to
complain about this, so lets just refactor the code to suppress the
warnings.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14043

module/zfs/spa.c

@@ -8867,9 +8867,6 @@ spa_sync_props(void *arg, dmu_tx_t *tx)
 				spa_history_log_internal(spa, "set", tx,
 				    "%s=%lld", nvpair_name(elem),
 				    (longlong_t)intval);
-			} else {
-				ASSERT(0); /* not allowed */
-			}
 
 				switch (prop) {
 				case ZPOOL_PROP_DELEGATION:
@@ -8898,6 +8895,9 @@ spa_sync_props(void *arg, dmu_tx_t *tx)
 				default:
 					break;
 				}
+			} else {
+				ASSERT(0); /* not allowed */
+			}
 		}
 
 	}