The 'level' of an md array can be set as either a number of a string.  When
one is set, the other must be marked 'undefined'.  This wasn't being done
in one place: where new arrays are created.

Result: if md1 is a raid1, it is stopped and a raid5 is created there, it
might still appear to be a raid1.
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>