Use setmntent with "e" option

Otherwise it will cause selinux warning in children processes sometimes

Change-Id: I41239c3f9779140622076c644a5f63051d00eaa8
diff --git a/VolumeManager.cpp b/VolumeManager.cpp
index 21136cf..59a3d1b 100644
--- a/VolumeManager.cpp
+++ b/VolumeManager.cpp
@@ -589,7 +589,7 @@
 
     // Worst case we might have some stale mounts lurking around, so
     // force unmount those just to be safe.
-    FILE* fp = setmntent("/proc/mounts", "r");
+    FILE* fp = setmntent("/proc/mounts", "re");
     if (fp == NULL) {
         PLOG(ERROR) << "Failed to open /proc/mounts";
         return -errno;