mm: faster kmalloc_array(), kcalloc()

When both arguments to kmalloc_array() or kcalloc() are known at compile
time then their product is known at compile time but search for kmalloc
cache happens at runtime not at compile time.

Link: http://lkml.kernel.org/r/20160627213454.GA2440@p183.telecom.by
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/slab.h

@@ -565,6 +565,8 @@ static inline void *kmalloc_array(size_t n, size_t size, gfp_t flags)
 {
 	if (size != 0 && n > SIZE_MAX / size)
 		return NULL;
+	if (__builtin_constant_p(n) && __builtin_constant_p(size))
+		return kmalloc(n * size, flags);
 	return __kmalloc(n * size, flags);
 }