nvme: fix NULL derefence in nvme_ctrl_fast_io_fail_tmo_show/store

Adding entry for dev_attr_fast_io_fail_tmo to avoid the kernel crash
while reading and writing the fast_io_fail_tmo.

Fixes: 09fbed63

 (nvme: export fast_io_fail_tmo to sysfs)
Signed-off-by: Gopal Tiwari <gtiwari@redhat.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>