mm/swap: fix race on swap_info reuse between swapoff and swapon

mm/swap: fix race on swap_info reuse between swapoff and swapon

swapoff clear swap_info's SWP_USED flag prematurely and free its
resources after that.  A concurrent swapon will reuse this swap_info
while its previous resources are not cleared completely.

These late freed resources are:
 - p->percpu_cluster
 - swap_cgroup_ctrl[type]
 - block_device setting
 - inode->i_flags &= ~S_SWAPFILE

This patch clears the SWP_USED flag after all its resources are freed,
so that swapon can reuse this swap_info by alloc_swap_info() safely.

[[email protected]: tidy up code comment]
Signed-off-by: Weijie Yang <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Cc: Krzysztof Kozlowski <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>