svcrpc: remove handling of unknown errors from svc_recv

svc_recv() returns only -EINTR or -EAGAIN.  If we really want to worry
about the case where it has a bug that causes it to return something
else, we could stick a WARN() in svc_recv.  But it's silly to require
every caller to have all this boilerplate to handle that case.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>