SUNRPC: improve error response to over-size gss credential

When the NFS server receives a large gss (kerberos) credential and tries
to pass it up to rpc.svcgssd (which is deprecated), it triggers an
infinite loop in cache_read().

cache_request() always returns -EAGAIN, and this causes a "goto again".

This patch:
 - changes the error to -E2BIG to avoid the infinite loop, and
 - generates a WARN_ONCE when rsi_request first sees an over-sized
   credential.  The warning suggests switching to gssproxy.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=196583


Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>