[PATCH] md: allow hot-adding devices to arrays with non-persistant superblocks.

It is possibly (and occasionally useful) to have a raid1 without persistent
superblocks.  The code in add_new_disk for adding a device to such an array
always tries to read a superblock.

This will obviously fail.

So do the appropriate test and call md_import_device with
appropriate args.

Signed-off-by: Neil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>