NFSD: only call fh_unlock() once in nfsd_link()

On non-error paths, nfsd_link() calls fh_unlock() twice.  This is safe
because fh_unlock() records that the unlock has been done and doesn't
repeat it.
However it makes the code a little confusing and interferes with changes
that are planned for directory locking.

So rearrange the code to ensure fh_unlock() is called exactly once if
fh_lock() was called.

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