blktrace: pdu_buf of pc events should be unsigned

I got this:
  8,0    1   305.417782332  2037  I   R 32 (ffffff9e 10 00 ...) [bash]

It should be:
  8,0    1   305.417782332  2037  I   R 32 (9e 10 00 ...) [bash]

[ Impact: fix output of pc events ]

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <4A07C6B3.9080802@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/blktrace.c

@@ -1065,7 +1065,7 @@ static int blk_log_action(struct trace_iterator *iter, const char *act)
 
 static int blk_log_dump_pdu(struct trace_seq *s, const struct trace_entry *ent)
 {
-	const char *pdu_buf;
+	const unsigned char *pdu_buf;
 	int pdu_len;
 	int i, end, ret;