include/linux/err.h: add a function to cast error-pointers to a return value

PTR_RET() can be used if you have an error-pointer and are only interested
in the eventual error value, but not the pointer.  Yields the usual 0 for
no error, -ESOMETHING otherwise.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/err.h

@@ -52,6 +52,14 @@ static inline void * __must_check ERR_CAST(const void *ptr)
 	return (void *) ptr;
 }
 
+static inline int __must_check PTR_RET(const void *ptr)
+{
+	if (IS_ERR(ptr))
+		return PTR_ERR(ptr);
+	else
+		return 0;
+}
+
 #endif
 
 #endif /* _LINUX_ERR_H */