ssa.c: Eliminate some unneeded parentheses in ssa_downstream_svc_server

Signed-off-by: Hal Rosenstock <hal@mellanox.com>
ssa.c: Handle EAGAIN/EWOULDBLOCK from rsend

Not an error so don't log anything.
Currently, retries immediately.

Signed-off-by: Hal Rosenstock <hal@mellanox.com>
ssa.c: Handle EAGAIN/EWOULDBLOCK from riowrite

Not an error so don't log anything.
Currently, retries immediately.

Signed-off-by: Hal Rosenstock <hal@mellanox.com>
ssa.c: Handle EAGAIN/EWOULDBLOCK from rrecv

Not an error so don't log anything.
Currently, retries immediately.

Signed-off-by: Hal Rosenstock <hal@mellanox.com>
ssa.c: Handle rrecv return value of 0 properly

rrecv return value 0 means that the rsocket has been disconnected

Right now, it just logs an error message.
Add rsocket closing and cleanup of pollfds in a future patch.

Signed-off-by: Hal Rosenstock <hal@mellanox.com>