[PATCH] kernel.h: fix new warnings for container_of()

[Arnd Bergmann]

I see new warnings with gcc-7.0.1 with the modified container_of():

fs/f2fs/dir.c: In function 'F2FS_I':
fs/f2fs/f2fs.h:1122:385: note: found mismatched ssa struct pointer types: 'struct f2fs_inode_info' and 'struct inode'

This seems to happen for all structures that have a zero offset
between the member and the container structure, i.e. idential
pointers.

Reverting to an intermediate pointer avoids the warning, and using
a void pointer instead of the target type should also avoid
regressing on the previous patch again.

Fixes: mmotm ("kernel.h: handle pointers to arrays better in container_of()")
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
---
 include/linux/kernel.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index d043adadcf33..bd6d96cf80b1 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -856,10 +856,11 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
  *
  */
 #define container_of(ptr, type, member) ({				\
+	void *__mptr = (void *)(ptr);					\
 	BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) &&	\
 			 !__same_type(*(ptr), void),			\
 			 "pointer type mismatch in container_of()");	\
-	((type *)((char *)(ptr) - offsetof(type, member))); })
+	((type *)(__mptr - offsetof(type, member))); })
 
 /* Rebuild everything on CONFIG_FTRACE_MCOUNT_RECORD */
 #ifdef CONFIG_FTRACE_MCOUNT_RECORD
-- 
2.9.0