btrfs: turn on -Wmaybe-uninitialized

We had a recent bug that would have been caught by a newer compiler with
-Wmaybe-uninitialized and would have saved us a month of failing tests
that I didn't have time to investigate.
Reviewed-by: NQu Wenruo <wqu@suse.com>
Signed-off-by: NJosef Bacik <josef@toxicpanda.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/Makefile

@@ -11,7 +11,8 @@ condflags := \
 	$(call cc-option, -Wunused-but-set-variable)		\
 	$(call cc-option, -Wunused-const-variable)		\
 	$(call cc-option, -Wpacked-not-aligned)			\
-	$(call cc-option, -Wstringop-truncation)
+	$(call cc-option, -Wstringop-truncation)		\
+	$(call cc-option, -Wmaybe-uninitialized)
 subdir-ccflags-y += $(condflags)
 # The following turn off the warnings enabled by -Wextra
 subdir-ccflags-y += -Wno-missing-field-initializers