Revert "Remove support for the MD linear RAID level"

This reverts commit a21c4fa.

The removal of the kernel module was reverted and apparently a lot
of people use it.

blivet/devicelibs/mdraid.py

@@ -45,6 +45,6 @@ def is_raid_level(cls, level):
             hasattr(level, 'get_size')
 
 
-raid_levels = MDRaidLevels(["raid0", "raid1", "raid4", "raid5", "raid6", "raid10"])
+raid_levels = MDRaidLevels(["raid0", "raid1", "raid4", "raid5", "raid6", "raid10", "linear"])
 
 EXTERNAL_DEPENDENCIES = [availability.BLOCKDEV_MDRAID_PLUGIN]