pstore: do not use message compression without lock

pstore_compress() uses static stream buffer for zlib-deflate which
easily crashes when several concurrent threads use one shared state.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Tony Luck <tony.luck@intel.com>

fs/pstore/platform.c

@@ -299,7 +299,7 @@ static void pstore_dump(struct kmsg_dumper *dumper,
 		bool compressed;
 		size_t total_len;
 
-		if (big_oops_buf) {
+		if (big_oops_buf && is_locked) {
 			dst = big_oops_buf;
 			hsize = sprintf(dst, "%s#%d Part%u\n", why,
 							oopscount, part);