staging: kernel.h: Prevent macro expantion bug in container_of_safe()

There aren't many users of this so it doesn't cause a problem, but we
obviously want to use "__mptr" here instead of "ptr" to prevent the
parameter from being executed twice.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/kernel.h

@@ -977,7 +977,7 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
 	BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) &&	\
 			 !__same_type(*(ptr), void),			\
 			 "pointer type mismatch in container_of()");	\
-	IS_ERR_OR_NULL(ptr) ? ERR_CAST(ptr) :				\
+	IS_ERR_OR_NULL(__mptr) ? ERR_CAST(__mptr) :			\
 		((type *)(__mptr - offsetof(type, member))); })
 
 /* Rebuild everything on CONFIG_FTRACE_MCOUNT_RECORD */