Squashfs: fix failure to unlock pages on decompress error

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: Roman Peniaev <r.peniaev@gmail.com>
Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>