docs/core-api/mm: fix return value descriptions in mm/

Many kernel-doc comments in mm/ have the return value descriptions
either misformatted or omitted at all which makes kernel-doc script
unhappy:

$ make V=1 htmldocs
...
./mm/util.c:36: info: Scanning doc for kstrdup
./mm/util.c:41: warning: No description found for return value of 'kstrdup'
./mm/util.c:57: info: Scanning doc for kstrdup_const
./mm/util.c:66: warning: No description found for return value of 'kstrdup_const'
./mm/util.c:75: info: Scanning doc for kstrndup
./mm/util.c:83: warning: No description found for return value of 'kstrndup'
...

Fixing the formatting and adding the missing return value descriptions
eliminates ~100 such warnings.

Link: http://lkml.kernel.org/r/1549549644-4903-4-git-send-email-rppt@linux.ibm.com
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/mempool.c b/mm/mempool.c
index 0ef8cc8..85efab3 100644
--- a/mm/mempool.c
+++ b/mm/mempool.c
@@ -222,6 +222,8 @@ EXPORT_SYMBOL(mempool_init_node);
  *
  * Like mempool_create(), but initializes the pool in (i.e. embedded in another
  * structure).
+ *
+ * Return: %0 on success, negative error code otherwise.
  */
 int mempool_init(mempool_t *pool, int min_nr, mempool_alloc_t *alloc_fn,
 		 mempool_free_t *free_fn, void *pool_data)
@@ -245,6 +247,8 @@ EXPORT_SYMBOL(mempool_init);
  * functions. This function might sleep. Both the alloc_fn() and the free_fn()
  * functions might sleep - as long as the mempool_alloc() function is not called
  * from IRQ contexts.
+ *
+ * Return: pointer to the created memory pool object or %NULL on error.
  */
 mempool_t *mempool_create(int min_nr, mempool_alloc_t *alloc_fn,
 				mempool_free_t *free_fn, void *pool_data)
@@ -289,6 +293,8 @@ EXPORT_SYMBOL(mempool_create_node);
  * Note, the caller must guarantee that no mempool_destroy is called
  * while this function is running. mempool_alloc() & mempool_free()
  * might be called (eg. from IRQ contexts) while this function executes.
+ *
+ * Return: %0 on success, negative error code otherwise.
  */
 int mempool_resize(mempool_t *pool, int new_min_nr)
 {
@@ -363,6 +369,8 @@ EXPORT_SYMBOL(mempool_resize);
  * *never* fails when called from process contexts. (it might
  * fail if called from an IRQ context.)
  * Note: using __GFP_ZERO is not supported.
+ *
+ * Return: pointer to the allocated element or %NULL on error.
  */
 void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
 {