mm, shmem: prevent infinite loop on truncate race

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

mm, shmem: prevent infinite loop on truncate race

When truncating a large swap entry, shmem_free_swap() returns 0 when the

entry's index doesn't match the given index due to lookup alignment. The

failure fallback path checks if the entry crosses the end border and

aborts when it happens, so truncate won't erase an unexpected entry or

range. But one scenario was ignored.

When `index` points to the middle of a large swap entry, and the large

swap entry doesn't go across the end border, find_get_entries() will

return that large swap entry as the first item in the batch with

`indices[0]` equal to `index`. The entry's base index will be smaller

than `indices[0]`, so shmem_free_swap() will fail and return 0 due to the

"base < index" check. The code will then call shmem_confirm_swap(), get

the order, check if it crosses the END boundary (which it doesn't), and

retry with the same index.

The next iteration will find the same entry again at the same index with

same indices, leading to an infinite loop.

Fix this by retrying with a round-down index, and abort if the index is

smaller than the truncate range.