pstore: Always allocate buffer for decompression

pstore: Always allocate buffer for decompression

Currently, pstore_mkfile() performs a memcpy() of the record contents,
so it can live anywhere. However, this is needlessly wasteful. In
preparation of pstore_mkfile() keeping the record contents, always
allocate a buffer for the contents.

Signed-off-by: Kees Cook <keescook@chromium.org>