target: rewrite fd_execute_write_same

With the new bio_vec backed iov_iter helpers we can simply set up
on bio_vec per LBA, all pointing to the same initiator-supplied
buffer.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>