efivarfs: Fix memory leak of efivarfs_fs_info in fs_context error paths

In the Linux kernel, the following vulnerability has been resolved:

efivarfs: Fix memory leak of efivarfs_fs_info in fs_context error paths

When processing mount options, efivarfs allocates efivarfs_fs_info (sfi) early in fs_context initialization. However, sfi is associated with the superblock and typically freed when the superblock is destroyed. If the fs_context is released (final put) before fill_super is called—such as on error paths or during reconfiguration—the sfi structure would leak, as ownership never transfers to the superblock.

Implement the .free callback in efivarfs_context_ops to ensure any allocated sfi is properly freed if the fs_context is torn down before fill_super, preventing this memory leak.