abd_return_buf() should call zfs_refcount_remove_many() early

Calling zfs_refcount_remove_many() after freeing memory means we pass a
reference to freed memory as the holder. This is not believed to be able
to cause a problem, but there is a bit of a tradition of fixing these
issues when they appear so that they do not obscure more serious issues
in static analyzer output, so we fix this one too.

Clang's static analyzer found this with the help of CodeChecker's CTU
analysis.

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

module/zfs/abd.c

@@ -667,15 +667,15 @@ abd_return_buf(abd_t *abd, void *buf, size_t n)
 {
 	abd_verify(abd);
 	ASSERT3U(abd->abd_size, >=, n);
+#ifdef ZFS_DEBUG
+	(void) zfs_refcount_remove_many(&abd->abd_children, n, buf);
+#endif
 	if (abd_is_linear(abd)) {
 		ASSERT3P(buf, ==, abd_to_buf(abd));
 	} else {
 		ASSERT0(abd_cmp_buf(abd, buf, n));
 		zio_buf_free(buf, n);
 	}
-#ifdef ZFS_DEBUG
-	(void) zfs_refcount_remove_many(&abd->abd_children, n, buf);
-#endif
 }
 
 void