[PATCH] uml ubd driver: document some struct fields

Add documentation about some fields in struct ubd, whose meaning is
non-obvious due to struct names (should change names altogether, I agree).

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>