first_stage_mount: Fix log typo "check_at_most_once"

Was "check_most_at_once", should be "check_at_most_once".
Also straighten the if-then-else logic from:
  if (cond) {
    // then
  } else {
    // error...
    return;
  }
to:
  if (!cond) {
    // error...
    return;
  }
  // fallthrough is "then"

Test: Presubmit
Change-Id: I85c1e94c47a727089374d5d05ecd40f4187b66a1
diff --git a/init/first_stage_mount.cpp b/init/first_stage_mount.cpp
index 4bbbc20..07ce458 100644
--- a/init/first_stage_mount.cpp
+++ b/init/first_stage_mount.cpp
@@ -507,16 +507,16 @@
         SaveRamdiskPathToSnapuserd();
     }
 
-    if (MountPartition(system_partition, false /* erase_same_mounts */)) {
-        if (dsu_not_on_userdata_ && fs_mgr_verity_is_check_at_most_once(*system_partition)) {
-            LOG(ERROR) << "check_most_at_once forbidden on external media";
-            return false;
-        }
-        SwitchRoot("/system");
-    } else {
+    if (!MountPartition(system_partition, false /* erase_same_mounts */)) {
         PLOG(ERROR) << "Failed to mount /system";
         return false;
     }
+    if (dsu_not_on_userdata_ && fs_mgr_verity_is_check_at_most_once(*system_partition)) {
+        LOG(ERROR) << "check_at_most_once forbidden on external media";
+        return false;
+    }
+
+    SwitchRoot("/system");
 
     return true;
 }