btrfs-progs: fix update-check.

srcpkgs/btrfs-progs/template

@@ -11,6 +11,8 @@ short_desc="Btrfs filesystem utilities"
 maintainer="Juan RP <xtraeme@gmail.com>"
 license="GPL-2"
 homepage="http://btrfs.wiki.kernel.org/"
+update_pattern=$pkgname' \K[\d+.]+'
+update_version="${_distver}"
 
 do_fetch() {
 	git clone -b v${_distver} git://git.kernel.org/pub/scm/linux/kernel/git/kdave/${pkgname}.git ${pkgname}-${version}