perf symbols: Convert lseek + read to pread

When dso_cache__read() is called, it reads data from the given offset
using lseek + normal read syscall.  It can be combined to a single pread
syscall.
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1422518843-25818-40-git-send-email-namhyung@kernel.org
[ Fixed it up when cherry picking it from the multi threaded patchkit ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/dso.c

@@ -532,12 +532,8 @@ dso_cache__read(struct dso *dso, u64 offset, u8 *data, ssize_t size)
 			break;
 
 		cache_offset = offset & DSO__DATA_CACHE_MASK;
-		ret = -EINVAL;
 
-		if (-1 == lseek(dso->data.fd, cache_offset, SEEK_SET))
-			break;
-
-		ret = read(dso->data.fd, cache->data, DSO__DATA_CACHE_SIZE);
+		ret = pread(dso->data.fd, cache->data, DSO__DATA_CACHE_SIZE, cache_offset);
 		if (ret <= 0)
 			break;