Direct decompression into the page cache.  If we fall back
to using an intermediate buffer (because we cannot grab all the
page cache pages) and we get a decompress fail, we forgot to
release the pages.
Reported-by: NRoman Peniaev <r.peniaev@gmail.com>
Signed-off-by: NPhillip Lougher <phillip@squashfs.org.uk>