mm/slub.c: include swab.h

mm/slub.c: include swab.h

commit 1b3865d016815cbd69a1879ca1c8a8901fda1072 upstream.

Fixes build with CONFIG_SLAB_FREELIST_HARDENED=y.

Hopefully.  But it's the right thing to do anwyay.

Fixes: 1ad53d9fa3f61 ("slub: improve bit diffusion for freelist ptr obfuscation")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=213417
Reported-by: <vannguye@cisco.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>