mm/zswap.c: change params from hidden to ro

The "compressor" and "enabled" params are currently hidden, this changes
them to read-only, so userspace can tell if zswap is enabled or not and
see what compressor is in use.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Cc: Vladimir Murzin <murzin.v@gmail.com>
Cc: Bob Liu <bob.liu@oracle.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Weijie Yang <weijie.yang@samsung.com>
Acked-by: Seth Jennings <sjennings@variantweb.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>