Btrfs: show subvol= and subvolid= in /proc/mounts

Btrfs: show subvol= and subvolid= in /proc/mounts

Orabug: 26088734

Now that we're guaranteed to have a meaningful root dentry, we can just
export seq_dentry() and use it in btrfs_show_options(). The subvolume ID
is easy to get and can also be useful, so put that in there, too.

Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Omar Sandoval <osandov@osandov.com>
Signed-off-by: Chris Mason <clm@fb.com>
(cherry picked from commit c8d3fe028f64054d75c72566efb9ecf3c75ee161)
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>