nfsd: make use of warning provided by refcount_t

refcount_t, by design, checks for unwanted situations and provides
warnings.  It is rarely useful to have explicit warnings with refcount
usage.

In this case we have an explicit warning if a refcount_t reaches zero
when decremented.  Simply using refcount_dec() will provide a similar
warning and also mark the refcount_t as saturated to avoid any possible
use-after-free.

This patch drops the warning and uses refcount_dec() instead of
refcount_dec_and_test().

Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>