[PATCH] md: create a MODULE_ALIAS for md corresponding to its block major number.

I just discovered this is needed for module auto-loading.

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>