Add notify_inval_inode and extend get_rootfs to return fs_idx for umount

[sidneychang]

This PR includes two related improvements to better support multi-filesystem
mounting scenarios on fuse, especially during unmount operations:

1. Add notify_inval_inode support in fusedev, enabling user space to
   explicitly notify the kernel to invalidate inode page caches. This helps
   clear caches properly after unmounting.

2. Extend the VFS method get_rootfs to return both the mounted backend
   filesystem (Arc) and its associated filesystem index
   (fs_idx). Although multi-filesystem mounting is already supported,
   returning fs_idx is necessary for proper unmount handling and cache
   invalidation.

The implementation follows fuse kernel and libfuse protocols and maintains
compatibility with existing patterns like notify_inval_entry.

[imeoer]

Thanks, LGTM!