fs_mgr: overlay: preserve errno android::gsi::IsGsiRunning()

Resulted in remount reporting an mount_all failure.

Test: adb-remount-test.sh
Bug: 126256072
Change-Id: Ib9567748e5c12b047e38869c07755eb741308df4
diff --git a/fs_mgr/fs_mgr_overlayfs.cpp b/fs_mgr/fs_mgr_overlayfs.cpp
index 40da36d..e61f588 100644
--- a/fs_mgr/fs_mgr_overlayfs.cpp
+++ b/fs_mgr/fs_mgr_overlayfs.cpp
@@ -769,9 +769,10 @@
 
     // in recovery, fastbootd, or gsi mode, not allowed!
     if (fs_mgr_access("/system/bin/recovery")) return true;
-    if (android::gsi::IsGsiRunning()) return true;
-
-    return false;
+    auto save_errno = errno;
+    auto ret = android::gsi::IsGsiRunning();
+    errno = save_errno;
+    return ret;
 }
 
 }  // namespace