md/raid5: separate out the easy conditions in need_this_block.

Some of the conditions in need_this_block have very straight
forward motivation.  Separate those out and document them.

Signed-off-by: NeilBrown <neilb@suse.de>
This commit is contained in:
NeilBrown 2015-02-02 11:37:59 +11:00
commit a79cfe12c6