oomd: switch system context parsing to use /proc/meminfo

Makes it easier in the next commits to unify on one way to read swap and
memory info.

src/shared/psi-util.c

@@ -35,8 +35,8 @@ int read_resource_pressure(const char *path, PressureType type, ResourcePressure
                 return -EINVAL;
 
         r = fopen_unlocked(path, "re", &f);
-         if (r < 0)
-                 return r;
+        if (r < 0)
+                return r;
 
         for (;;) {
                 _cleanup_free_ char *l = NULL;