srpt: use target_execute_cmd for WRITEs in srpt_handle_rdma_comp

srpt_handle_rdma_comp is called from kthread context and thus can execute
target_execute_cmd directly.  srpt_abort_cmd sets the CMD_T_LUN_STOP
flag directly, and thus the abuse of transport_generic_handle_data can be
replaced with an opencoded variant of that code path.  I'm still not happy
about a fabric driver poking into target core internals like this, but
let's defer the bigger architecture changes for now.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>