Now parse_early_options() is used by both btrfs_mount() and
btrfs_mount_root(). However, the former only needs subvol related part
and the latter needs the others.

Therefore extract the subvol related parts from parse_early_options() and
move it to new parse function (parse_subvol_options()).
Signed-off-by: NTomohiro Misono <misono.tomohiro@jp.fujitsu.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>