swap: make each swap partition have one address_space

swap: make each swap partition have one address_space

When I use several fast SSD to do swap, swapper_space.tree_lock is heavily
contended.  This makes each swap partition have one address_space to
reduce the lock contention.  There is an array of address_space for swap.
The swap entry type is the index to the array.

In my test with 3 SSD, this increases the swapout throughput 20%.

Signed-off-by: Shaohua Li <shli@fusionio.com>
Cc: Hugh Dickins <hughd@google.com>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>