mm, swap: fix SWAP_USAGE_OFFLIST_BIT collision with real usage count

In the Linux kernel, the following vulnerability has been resolved:

mm, swap: fix SWAP_USAGE_OFFLIST_BIT collision with real usage count

SWAP_USAGE_OFFLIST_BIT is embedded in the si->inuse_pages usage counter,
and is meant to sit above any value that counter can reach. However, it
is defined from BITS_PER_TYPE(atomic_t), so it is bit 30. On a system
with 4 KiB pages the flag collides with the usage count once that count
reaches 4 TiB.

swap_usage_in_pages() masks bit 30 out, so whenever the real count has
that bit set, every caller of it reads 4 TiB low:

* /proc/swaps understates Used by 4 TiB.

* A raw count of exactly 2^30 masks to zero, so try_to_unuse() takes its
"if (!swap_usage_in_pages(si)) goto success;" early exit and swapoff
tears the device down while pages are still swapped out. Nothing in
the rest of swapoff aborts the teardown, so those pages are lost.

Independently of swapoff, the collision also corrupts the counter and the
plist. On a device in normal use, a free that leaves bit 30 set in the
count makes swap_usage_sub() see the flag where there is only count, and
call add_to_avail_list(). It clears the bit with
fetch_and(~SWAP_USAGE_OFFLIST_BIT), leaving the stored count 4 TiB below
the real one, and calls plist_add() on a device that is already listed,
tripping the WARN_ON(!plist_node_empty(node)) in plist_add() and linking
the node a second time.

Change the definition of SWAP_USAGE_OFFLIST_BIT to be based on
atomic_long_t instead. Note that the usage counter field itself is of
this same type, so it is still a valid bit.