ubuntu-cpc: set default ext4 mount option commit=30

This allows to consolidate linux-kvm and linux-generic kernel
flavours. This brings the perfomance benefit of linux-kvm flavour to
all cloud and pre-installed images. It does trade data-safety.

LP: #2006511

Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>

live-build/ubuntu-cpc/hooks.d/chroot/999-cpc-fixes.chroot

@@ -72,7 +72,7 @@ if [ -n "${root_fs_label}" ]; then
    sed -i "s,^[^#${bl}]*\([${bl}]*/[${bl}].*\),${lstr}\1," "${rootd}/etc/fstab"
 fi
 cat > /etc/fstab << EOM
-LABEL=cloudimg-rootfs	/	 ext4	discard,errors=remount-ro	0 1
+LABEL=cloudimg-rootfs	/	 ext4	discard,commit=30,errors=remount-ro	0 1
 EOM
 
 # for quantal and newer, add /etc/overlayroot.local.conf