mm/slab: Add __free() support for kvfree

Allow for the declaration of variables that trigger kvfree() when they
go out of scope. The check for NULL and call to kvfree() can be elided
by the compiler in most cases, otherwise without the NULL check an
unnecessary call to kvfree() may be emitted. Peter proposed a comment
for this detail [1].

Link: http://lore.kernel.org/r/[e-mail] [1]
Cc: Andrew Morton <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Acked-by: Pankaj Gupta <[e-mail]>
Acked-by: Greg Kroah-Hartman <[e-mail]>
Reviewed-by: Kuppuswamy Sathyanarayanan <[e-mail]>
Tested-by: Kuppuswamy Sathyanarayanan <[e-mail]>
Reviewed-by: Tom Lendacky <[e-mail]>
Signed-off-by: Dan Williams <[e-mail]>

include/linux/slab.h

···
 extern void *kvrealloc(const void *p, size_t oldsize, size_t newsize, gfp_t flags)
 		      __realloc_size(3);
 extern void kvfree(const void *addr);
+DEFINE_FREE(kvfree, void *, if (_T) kvfree(_T))
+
 extern void kvfree_sensitive(const void *addr, size_t len);
 
 unsigned int kmem_cache_size(struct kmem_cache *s);