move get_persistent_dev to dracut-functions.sh

move get_persistent_dev to dracut-functions.sh

kdump module also need to convert dev name to udev symlinks.
So better to move function get_persistent_dev() to dracut-functions.sh

Also in this patch improvement and fix the original function:
a) use udevadm info --query=name to get the kernel name.
   This will fix the issue caused by passing symbolic link of a device.
b) fix a bug to compare $_tmp instead of $i with $_dev. Really sorry,
   should have tested more carefully.

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