nvme-tcp: fix use-after-free of netns by kernel TCP socket.

Commit 1be52169 ("nvme-tcp: fix selinux denied when calling
sock_sendmsg") converted sock_create() in nvme_tcp_alloc_queue()
to sock_create_kern().

sock_create_kern() creates a kernel socket, which does not hold
a reference to netns.  If the code does not manage the netns
lifetime properly, use-after-free could happen.

Also, TCP kernel socket with sk_net_refcnt 0 has a socket leak
problem: it remains FIN_WAIT_1 if it misses FIN after close()
because tcp_close() stops all timers.

To fix such problems, let's hold netns ref by sk_net_refcnt_upgrade().

We had the same issue in CIFS, SMC, etc, and applied the same
solution, see commit ef7134c7 ("smb: client: Fix use-after-free
of network namespace.") and commit 9744d2bf ("smc: Fix
use-after-free in tcp_write_timer_handler().").

Fixes: 1be52169 ("nvme-tcp: fix selinux denied when calling sock_sendmsg")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>