kernel.h: documentation for roundup() vs round_up()

Things like 3619dec5 ("dh key: fix rounding up KDF output length")
expose the lack of explicit documentation for roundup() vs round_up().  At
least we can try to document it better if anyone goes looking.

Link: http://lkml.kernel.org/r/20180703041950.GA43464@beastSigned-off-by: Kees Cook <keescook@chromium.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/kernel.h

@@ -85,7 +85,23 @@
  * arguments just once each.
  */
 #define __round_mask(x, y) ((__typeof__(x))((y)-1))
+/**
+ * round_up - round up to next specified power of 2
+ * @x: the value to round
+ * @y: multiple to round up to (must be a power of 2)
+ *
+ * Rounds @x up to next multiple of @y (which must be a power of 2).
+ * To perform arbitrary rounding up, use roundup() below.
+ */
 #define round_up(x, y) ((((x)-1) | __round_mask(x, y))+1)
+/**
+ * round_down - round down to next specified power of 2
+ * @x: the value to round
+ * @y: multiple to round down to (must be a power of 2)
+ *
+ * Rounds @x down to next multiple of @y (which must be a power of 2).
+ * To perform arbitrary rounding down, use rounddown() below.
+ */
 #define round_down(x, y) ((x) & ~__round_mask(x, y))
 
 /**
@@ -110,13 +126,30 @@
 # define DIV_ROUND_UP_SECTOR_T(ll,d) DIV_ROUND_UP(ll,d)
 #endif
 
-/* The `const' in roundup() prevents gcc-3.3 from calling __divdi3 */
+/**
+ * roundup - round up to the next specified multiple
+ * @x: the value to up
+ * @y: multiple to round up to
+ *
+ * Rounds @x up to next multiple of @y. If @y will always be a power
+ * of 2, consider using the faster round_up().
+ *
+ * The `const' here prevents gcc-3.3 from calling __divdi3
+ */
 #define roundup(x, y) (					\
 {							\
 	const typeof(y) __y = y;			\
 	(((x) + (__y - 1)) / __y) * __y;		\
 }							\
 )
+/**
+ * rounddown - round down to next specified multiple
+ * @x: the value to round
+ * @y: multiple to round down to
+ *
+ * Rounds @x down to next multiple of @y. If @y will always be a power
+ * of 2, consider using the faster round_down().
+ */
 #define rounddown(x, y) (				\
 {							\
 	typeof(x) __x = (x);				\