nfsd: when updating an entry with RC_NOCACHE, just free it

There's no need to keep entries around that we're declaring RC_NOCACHE.
Ditto if there's a problem with the entry.

With this change too, there's no need to test for RC_UNUSED in the
search function. If the entry's in the hash table then it's either
INPROG or DONE.

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