[PATCH] md: assorted minor md/raid1 fixes

[PATCH] md: assorted minor md/raid1 fixes

1/ rationalise read_balance and "map" in raid1.  Discard map and
   tidyup the interface to read_balance so it can be used instead.

2/ use offsetof rather than a caclulation to find the size of an
   structure with a var-length array at the end.

3/ remove some meaningless #defines

4/ use printk_ratelimit to limit reports of failed sectors being redirected.

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>