IB/srpt: Fix srpt_handle_cmd() error paths

The target core function that should be called if target_submit_cmd()
fails is target_put_sess_cmd(). Additionally, change the return type
of srpt_handle_cmd() from int into void.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Estrin <alex.estrin@intel.com>
Cc: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>