err.h: add __must_check to error pointer handlers

Add __must_check to error pointer handlers to have the compiler warn about
mistakes like:

	if (err)
		ERR_PTR(err);

It found two bugs:

Mar 12 Nikula Jani [PATCH] enclosure: fix error path - actually return ERR_PTR() on error
Mar 12 Nikula Jani [PATCH] sunrpc: fix error path - actually return ERR_PTR() on error
Signed-off-by: Jani Nikula <ext-jani.1.nikula@nokia.com>
Cc: Phil Carmody <ext-phil.2.carmody@nokia.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/err.h

@@ -19,22 +19,22 @@
 
 #define IS_ERR_VALUE(x) unlikely((x) >= (unsigned long)-MAX_ERRNO)
 
-static inline void *ERR_PTR(long error)
+static inline void * __must_check ERR_PTR(long error)
 {
 	return (void *) error;
 }
 
-static inline long PTR_ERR(const void *ptr)
+static inline long __must_check PTR_ERR(const void *ptr)
 {
 	return (long) ptr;
 }
 
-static inline long IS_ERR(const void *ptr)
+static inline long __must_check IS_ERR(const void *ptr)
 {
 	return IS_ERR_VALUE((unsigned long)ptr);
 }
 
-static inline long IS_ERR_OR_NULL(const void *ptr)
+static inline long __must_check IS_ERR_OR_NULL(const void *ptr)
 {
 	return !ptr || IS_ERR_VALUE((unsigned long)ptr);
 }
@@ -46,7 +46,7 @@ static inline long IS_ERR_OR_NULL(const void *ptr)
  * Explicitly cast an error-valued pointer to another pointer type in such a
  * way as to make it clear that's what's going on.
  */
-static inline void *ERR_CAST(const void *ptr)
+static inline void * __must_check ERR_CAST(const void *ptr)
 {
 	/* cast away the const */
 	return (void *) ptr;