zram: use u32 for entry ac_time tracking

zram: use u32 for entry ac_time tracking

We can reduce sizeof(zram_table_entry) on 64-bit systems by converting
flags and ac_time to u32.  Entry flags fit into u32, and for ac_time u32
gives us over a century of entry lifespan (approx 136 years) which is
plenty (zram uses system boot time (seconds)).

In struct zram_table_entry we use bytes aliasing, because bit-wait API
(for slot lock) requires a whole unsigned long word.

Link: https://lkml.kernel.org/r/d7c0b48450c70eeb5fd8acd6ecd23593f30dbf1f.1765775954.git.senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Suggested-by: David Stevens <stevensd@google.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Minchan Kim <minchan@google.com>
Cc: Richard Chang <richardycc@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>