chore: fix new clang-tidy warnings

The new version of `clang-tidy` has some new warnings:
* Fix warning about uninitialized variables by initializing them to 0.
* Fix warning about name starting with `__` (which is reserved for the
  standard library headers) by renaming to start with a single
  underscore.
* Globally disable `readability-math-missing-parentheses` because it is
  triggered too eagerly.

Also, the "999 warnings generated" messages are no longer printed to
stderr, so they cannot be ignored by redirecting stderr to `/dev/null`.
Instead, filter them out with `grep -v`.

Change-Id: I5a80aa85a4e37206f7eaa4130ffa1f40eb9e6dc4
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/src/ffa_memory.c b/src/ffa_memory.c
index ed9fe17..3ec860c 100644
--- a/src/ffa_memory.c
+++ b/src/ffa_memory.c
@@ -1474,7 +1474,7 @@
 	uint32_t j;
 	mm_mode_t orig_from_mode;
 	mm_mode_t clean_mode;
-	mm_mode_t from_mode;
+	mm_mode_t from_mode = 0;
 	struct mpool local_page_pool;
 	struct ffa_value ret;
 	uint32_t constituents_total_page_count = 0;
@@ -1758,7 +1758,7 @@
 {
 	mm_mode_t orig_from_mode;
 	mm_mode_t clearing_mode;
-	mm_mode_t from_mode;
+	mm_mode_t from_mode = 0;
 	struct mpool local_page_pool;
 	struct ffa_value ret;
 	enum ffa_map_action map_action;