mm: zswap: fix missing folio cleanup in writeback race path

mm: zswap: fix missing folio cleanup in writeback race path

In zswap_writeback_entry(), after we get a folio from
__read_swap_cache_async(), we grab the tree lock again to check that the
swap entry was not invalidated and recycled.  If it was, we delete the
folio we just added to the swap cache and exit.

However, __read_swap_cache_async() returns the folio locked when it is
newly allocated, which is always true for this path, and the folio is
ref'd.  Make sure to unlock and put the folio before returning.

This was discovered by code inspection, probably because this path handles
a race condition that should not happen often, and the bug would not crash
the system, it will only strand the folio indefinitely.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 04fc7816089c ("mm: fix zswap writeback race condition")
Signed-off-by: Yosry Ahmed <[email protected]>
Reviewed-by: Chengming Zhou <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Reviewed-by: Nhat Pham <[email protected]>
Cc: Domenico Cerasuolo <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>