svcrpc: free contexts immediately on PROC_DESTROY

We currently handle a client PROC_DESTROY request by turning it
CACHE_NEGATIVE, setting the expired time to now, and then waiting for
cache_clean to clean it up later.  Since we forgot to set the cache's
nextcheck value, that could take up to 30 minutes.  Also, though there's
probably no real bug in this case, setting CACHE_NEGATIVE directly like
this probably isn't a great idea in general.

So let's just remove the entry from the cache directly, and move this
bit of cache manipulation to a helper function.

Signed-off-by: Neil Brown <neilb@suse.com>
Reported-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Andy Adamson <andros@netapp.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>