Btrfs: introduce btrfs_show_options

btrfs options can change at times other than mount, yet /proc/mounts shows the
options string used when the fs was mounted (an example would be when btrfs
determines that barriers aren't useful and turns them off.)  This patch
instead outputs the actual options in use by btrfs.

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>