mm: cachestat: fix two shmem bugs

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

mm: cachestat: fix two shmem bugs

When cachestat on shmem races with swapping and invalidation, there
are two possible bugs:

1) A swapin error can have resulted in a poisoned swap entry in the
shmem inode's xarray. Calling get_shadow_from_swap_cache() on it
will result in an out-of-bounds access to swapper_spaces[].

Validate the entry with non_swap_entry() before going further.

2) When we find a valid swap entry in the shmem's inode, the shadow
entry in the swapcache might not exist yet: swap IO is still in
progress and we're before __remove_mapping; swapin, invalidation,
or swapoff have removed the shadow from swapcache after we saw the
shmem swap entry.

This will send a NULL to workingset_test_recent(). The latter
purely operates on pointer bits, so it won't crash - node 0, memcg
ID 0, eviction timestamp 0, etc. are all valid inputs - but it's a
bogus test. In theory that could result in a false "recently
evicted" count.

Such a false positive wouldn't be the end of the world. But for
code clarity and (future) robustness, be explicit about this case.

Bail on get_shadow_from_swap_cache() returning NULL.