9p/rdma: do not disconnect on down_interruptible EAGAIN

9p/rdma would sometimes drop the connection and display errors in
recv_done when the user does ^C.
The errors were caused by recv buffers that were posted at the time
of disconnect, and we just do not want to disconnect when
down_interruptible is... interrupted.

Link: http://lkml.kernel.org/r/1535625307-18019-1-git-send-email-asmadeus@codewreck.org
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>