IB/srpt: Fix srpt_write_pending()

The only allowed return values for the write_pending() callback
function are 0, -EAGAIN and -ENOMEM. Since attempting to perform
RDMA over a disconnecting channel will result in an IB error
completion anyway, remove the code that checks the channel state
from srpt_write_pending().

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