cifs: avoid NULL pointer dereference in dbg call

commit b4885bd5 upstream.

cifs_server_dbg() implies server to be non-NULL so
move call under condition to avoid NULL pointer dereference.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: e79b0332 ("cifs: ignore cached share root handle closing errors")
Cc: stable@vger.kernel.org
Signed-off-by: Alexandra Diupina <adiupina@astralinux.ru>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>