nfsd: fix cred leak on every rpc

nfsd: fix cred leak on every rpc

Since override_creds() took its own reference on new, we need to release
our own reference.

(Note the put_cred on the return value puts the *old* value of
current->creds, not the new passed-in value).

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>