[PATCH] md: allow md to load a superblock with feature-bit '1' set

As this is used to flag an internal bitmap.

Also, introduce symbolic names for feature bits.

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>