dm-crypt: add missing error handling

There were cases where error code would be used only if we finish
processing the last sector. If there were other encryption operations
in flight, error would be ignored and bio would be returned with
success as if no error happened.

This bug is present in kcryptd_crypt_write_convert,
kcryptd_crypt_read_convert and kcryptd_async_done.

This patch fixes it, setting io->error to -EIO always when an
error happens.

Change-Id: I4aa7cbebcccff85bdbf9ad9bb70f6f52fc104afe
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Shashank Mittal <mittals@codeaurora.org>