mm/swap_slots.c: use conditional compilation

For mm/swap_slots.c, use the traditional Linux method of conditional
compilation and linking instead of always compiling it by using #ifdef
CONFIG_SWAP and #endif for the entire source file (excluding header
files).

Link: http://lkml.kernel.org/r/c2a47015-0b5a-d0d9-8bc7-9984c049df20@infradead.org


Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Tim Chen <tim.c.chen@linux.intel.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>