Refactor dmu_req_copy for immutable biovec changes

Originally, dmu_req_copy modifies bv_len and bv_offset in bio_vec so that it
can continue in subsequent passes. However, after the immutable biovec changes
in Linux 3.14, this is not allowed. So instead, we just tell dmu_req_copy how
many bytes are already copied and it will skip to the right spot accordingly.

Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Revert "Fix zvol+btrfs hang"

After the dmu_req_copy change, bi_io_vecs are not touched, so this is no
longer needed.

This reverts commit e26ade5101ba1d8e8350ff1270bfca4258e1ffe3.

Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Linux 3.14 compat: rq_for_each_segment in dmu_req_copy

rq_for_each_segment changed from taking bio_vec * to taking bio_vec.
We provide rq_for_each_segment4 which takes both.

Signed-off-by: Chunwei Chen <tuxoko@gmail.com>