fs_mgr: Fix return in error case

We were doing "return -1" in an error case from a function that
returns bool.  Since it's non-zero, this gets implicitly cast to
'true', which almost certainly isn't what we wanted here.

Test: TreeHugger
Change-Id: I76c188c22d907fc425cf4d5b0b1575f640a78ee3
diff --git a/fs_mgr/fs_mgr.cpp b/fs_mgr/fs_mgr.cpp
index 48ce4cd..15c9dfb 100644
--- a/fs_mgr/fs_mgr.cpp
+++ b/fs_mgr/fs_mgr.cpp
@@ -1370,7 +1370,7 @@
         Fstab proc_mounts;
         if (!ReadFstabFromFile("/proc/mounts", &proc_mounts)) {
             LERROR << "Can't read /proc/mounts";
-            return -1;
+            return false;
         }
         // Now proceed with other bind mounts on top of /data.
         for (const auto& entry : proc_mounts) {