btrfs: fix build when using M=fs/btrfs

There are people building the module with M= that's supposed to be used
for external modules. This got broken in e9aa7c285d20 ("btrfs: enable
W=1 checks for btrfs").

  $ make M=fs/btrfs
  scripts/Makefile.lib:10: *** Recursive variable 'KBUILD_CFLAGS' references itself (eventually).  Stop.
  make: *** [Makefile:1755: modules] Error 2

There's a difference compared to 'make fs/btrfs/btrfs.ko' which needs
to rebuild a few more things and also the dependency modules need to be
available. It could fail with eg.

  WARNING: Symbol version dump "Module.symvers" is missing.
	   Modules may not have dependencies or modversions.

In some environments it's more convenient to rebuild just the btrfs
module by M= so let's make it work.

The problem is with recursive variable evaluation in += so the
conditional C options are stored in a temporary variable to avoid the
recursion.

Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/Makefile

@@ -7,10 +7,12 @@ subdir-ccflags-y += -Wmissing-format-attribute
 subdir-ccflags-y += -Wmissing-prototypes
 subdir-ccflags-y += -Wold-style-definition
 subdir-ccflags-y += -Wmissing-include-dirs
-subdir-ccflags-y += $(call cc-option, -Wunused-but-set-variable)
-subdir-ccflags-y += $(call cc-option, -Wunused-const-variable)
-subdir-ccflags-y += $(call cc-option, -Wpacked-not-aligned)
-subdir-ccflags-y += $(call cc-option, -Wstringop-truncation)
+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)
+subdir-ccflags-y += $(condflags)
 # The following turn off the warnings enabled by -Wextra
 subdir-ccflags-y += -Wno-missing-field-initializers
 subdir-ccflags-y += -Wno-sign-compare