svcrpc: make xpo_recvfrom return only >=0

The only errors returned from xpo_recvfrom have been -EAGAIN and
-EAFNOSUPPORT.  The latter was removed by a previous patch.  That leaves
only -EAGAIN, which is treated just like 0 by the caller (svc_recv).

So, just ditch -EAGAIN and return 0 instead.

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