nfsd: simplify put of fi_deleg_file

fi_delegees is basically just a reference count on users of
fi_deleg_file, which is cleared when fi_delegees goes to zero.  The
fi_deleg_file check here is redundant.  Also add an assertion to make
sure we don't have unbalanced puts.

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