Set desc_nr more sanely.

Currently rdev->desc_nr is set in sync_sbs which is typcially
called just before writing out the superblocks, which is an
odd place to set it.
It is also called when a new disk is added (which is sane) and
when an old disc is imported ... which is quesitonable.

With this patch it is set when a new disk is added, and when
the superblocks are being analysed, which makes lots of sense.

MULTIPATH is particularly an issue here.  The old code tried
to figure the desc_nr for an rdev by matching device numbers in
the superblock.  This doesn't make a lot of sense as
device numbers can change.  Now MULTIPATH components
get sequential desc_nrs.