btrfs: convert zstd_decompress() to take a folio

btrfs: convert zstd_decompress() to take a folio

The old page API is being gradually replaced and converted to use folio
to improve code readability and avoid repeated conversion between page
and folio. And memcpy_to_page() can be replaced with memcpy_to_folio().
But there is no memzero_folio(), but it can be replaced equivalently by
folio_zero_range().

Signed-off-by: Li Zetao <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>