btrfs: zlib: fix the folio leak on S390 hardware acceleration

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

btrfs: zlib: fix the folio leak on S390 hardware acceleration

[BUG]
After commit aa60fe12b4f4 ("btrfs: zlib: refactor S390x HW acceleration
buffer preparation"), we no longer release the folio of the page cache
of folio returned by btrfs_compress_filemap_get_folio() for S390
hardware acceleration path.

[CAUSE]
Before that commit, we call kumap_local() and folio_put() after handling
each folio.

Although the timing is not ideal (it release previous folio at the
beginning of the loop, and rely on some extra cleanup out of the loop),
it at least handles the folio release correctly.

Meanwhile the refactored code is easier to read, it lacks the call to
release the filemap folio.

[FIX]
Add the missing folio_put() for copy_data_into_buffer().