fuse: add subvol_root_ino mount option for subvolume support

[ukernel]

Filesystems that support snapshots and subvolumes may have inodes with the same inode number across different subvolumes. When these inodes are accessed through the same FUSE mount, tools like cp(1) can incorrectly report "File X and Y are the same file" because they share the same device number (sb->s_dev).

Add a new mount option "subvol_root_ino" that specifies the inode number of snapshot/subvolume root. When set, FUSE allocates unique anonymous block device ID for inodes whose orig_ino matches this value, and all inodes under that subvolume inherit the same device ID. This gives each subvolume a distinct device number in stat(2), avoiding inode number conflicts.

The feature is implemented by:

• Adding sub_dev to fuse_inode, to track per-subvolume device ID
• Adding subvol_root_ino to fuse_conn
• Extending fuse_iget and fuse_lookup_name to propagate sub_dev
• Reporting fi->sub_dev in fuse_fillattr/fuse_getattr when set
• Disabling NFS export_support when subvol_root_ino is enabled, since the two features are incompatible

[ukernel]

the reason we don't use submount feature is that:
we want to support very large number of snapshots and linkable-datasets. Too many mounts will flood /proc/mounts and add heavy burden to VFS.