SUNRPC: Enforce an upper limit on the number of cached credentials

In some cases where the credentials are not often reused, we may want
to limit their total number just in order to make the negative lookups
in the hash table more manageable.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

diff --cc net/sunrpc/auth.c
Simple merge