add default values in fstab_lines

add default values in fstab_lines

It's useful for passing a full fstab line including like fs_passno so fsck
can take effect.

Previously it's assumed that there's no fs_freq and fs_passno in fstab lines
so original code just append "0 0" at the end of each fstab lines.

Improve this issue by assign default value in case they are not passed in.
Three field are handled here:
fs_mntops: default to "defaults"
fs_freq: default to "0"
fs_passno: default to "2"

Signed-off-by: Dave Young <dyoung@redhat.com>