Use setmntent with "e" option

Otherwise it will cause selinux warning in children processes sometimes

Change-Id: I41239c3f9779140622076c644a5f63051d00eaa8

Utils.cpp

@@ -766,7 +766,7 @@ bool IsRunningInEmulator() {
 }
 
 status_t UnmountTree(const std::string& prefix) {
-    FILE* fp = setmntent("/proc/mounts", "r");
+    FILE* fp = setmntent("/proc/mounts", "re");
     if (fp == NULL) {
         PLOG(ERROR) << "Failed to open /proc/mounts";
         return -errno;

VolumeManager.cpp

@@ -589,7 +589,7 @@ int VolumeManager::unmountAll() {
 
     // 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;