powerpc/pseries: Support compression of oops text via pstore

The patch set supports compression of oops messages while writing to NVRAM,
this helps in capturing more of oops data to lnx,oops-log. The pstore file
for oops messages will be in decompressed format making it readable.

In case compression fails, the patch takes care of copying the header added
by pstore and last oops_data_sz bytes of big_oops_buf to NVRAM so that we
have recent oops messages in lnx,oops-log.

In case decompression fails, it will result in absence of oops file but still
have files (in /dev/pstore) for other partitions.

Signed-off-by: Aruna Balakrishnaiah <aruna@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>