md: raid10: refactor code of read reshape's .bi_end_io

reshape read request is a bit special and requires one extra
bio which isn't allocated from r10buf_pool.

Refactor the .bi_end_io for read reshape, so that we can use
raid10's resync page mangement approach easily in the following
patches.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Shaohua Li <shli@fb.com>