Give --persistent_policy precedence over /dev/mapper names

Give --persistent_policy precedence over /dev/mapper names

There is currently no way to override dracut's preference for
/dev/mapper device names. But using these is problematic in
different scenarios: For example, if a user has a multipath-
enabled system but wants to disable multipath, or if the
names of multipath maps change because of configuration changes
(e.g. toggling user_friendly_names in /etc/multipath.conf).

This patch makes dracut prefer the user-specified
--persistent_policy names over /dev/mapper names.

It might be worthwhile to discuss why dracut prefers /dev/mapper
of /dev/disk/by-uuid at all. This preference was introduced
in 9037b63e with the argument "dm devices maintain /dev/mapper/* as
persistent names", but that's wrong for the scenarios mentioned
above, and is not a compelling reason for preferring /dev/mapper
over /dev/disk/by-uuid.

References: bsc#908143

Signed-off-by: Martin Wilck <mwilck@suse.de>