fs_mgr fix flags/fs_mgr_flags mismatch

Mount flags prefixed with MS_ are intended for the 'flags' entry of
fstab_rec, not fs_mgr_flags.

Coincidentally, this probably has always worked since this maps to the
'wait' flag, which is almost always set for system in the first place.

Test: build
Change-Id: Id4862bc087aa98e1f0e17145dc3ed74b5d867596
diff --git a/fs_mgr/fs_mgr.cpp b/fs_mgr/fs_mgr.cpp
index 820ff31..1631c8d 100644
--- a/fs_mgr/fs_mgr.cpp
+++ b/fs_mgr/fs_mgr.cpp
@@ -1002,7 +1002,7 @@
         /* Skip mounting the root partition, as it will already have been mounted */
         if (!strcmp(fstab->recs[i].mount_point, "/") ||
             !strcmp(fstab->recs[i].mount_point, "/system")) {
-            if ((fstab->recs[i].fs_mgr_flags & MS_RDONLY) != 0) {
+            if ((fstab->recs[i].flags & MS_RDONLY) != 0) {
                 fs_mgr_set_blk_ro(fstab->recs[i].blk_device);
             }
             continue;