SUNRPC/TLS: Lock the lower_xprt during the tls handshake

SUNRPC/TLS: Lock the lower_xprt during the tls handshake

Otherwise we run the risk of having the lower_xprt freed from underneath
us, causing an oops that looks like this:

[  224.150698] BUG: kernel NULL pointer dereference, address: 0000000000000018
[  224.150951] #PF: supervisor read access in kernel mode
[  224.151117] #PF: error_code(0x0000) - not-present page
[  224.151278] PGD 0 P4D 0
[  224.151361] Oops: 0000 [#1] PREEMPT SMP NOPTI
[  224.151499] CPU: 2 PID: 99 Comm: kworker/u10:6 Not tainted 6.6.0-rc3-g6465e260f487 #41264 a00b0960990fb7bc6d6a330ee03588b67f08a47b
[  224.151977] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS unknown 2/2/2022
[  224.152216] Workqueue: xprtiod xs_tcp_tls_setup_socket [sunrpc]
[  224.152434] RIP: 0010:xs_tcp_tls_setup_socket+0x3cc/0x7e0 [sunrpc]
[  224.152643] Code: 00 00 48 8b 7c 24 08 e9 f3 01 00 00 48 83 7b c0 00 0f 85 d2 01 00 00 49 8d 84 24 f8 05 00 00 48 89 44 24 10 48 8b 00 48 89 c5 <4c> 8b 68 18 66 41 83 3f 0a 75 71 45 31 ff 4c 89 ef 31 f6 e8 5c 76
[  224.153246] RSP: 0018:ffffb00ec060fd18 EFLAGS: 00010246
[  224.153427] RAX: 0000000000000000 RBX: ffff8c06c2e53e40 RCX: 0000000000000001
[  224.153652] RDX: ffff8c073bca2408 RSI: 0000000000000282 RDI: ffff8c06c259ee00
[  224.153868] RBP: 0000000000000000 R08: ffffffff9da55aa0 R09: 0000000000000001
[  224.154084] R10: 00000034306c30f1 R11: 0000000000000002 R12: ffff8c06c2e51800
[  224.154300] R13: ffff8c06c355d400 R14: 0000000004208160 R15: ffff8c06c2e53820
[  224.154521] FS:  0000000000000000(0000) GS:ffff8c073bd00000(0000) knlGS:0000000000000000
[  224.154763] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  224.154940] CR2: 0000000000000018 CR3: 0000000062c1e000 CR4: 0000000000750ee0
[  224.155157] PKRU: 55555554
[  224.155244] Call Trace:
[  224.155325]  <TASK>
[  224.155395]  ? __die_body+0x68/0xb0
[  224.155507]  ? page_fault_oops+0x34c/0x3a0
[  224.155635]  ? _raw_spin_unlock_irqrestore+0xe/0x40
[  224.155793]  ? exc_page_fault+0x7a/0x1b0
[  224.155916]  ? asm_exc_page_fault+0x26/0x30
[  224.156047]  ? xs_tcp_tls_setup_socket+0x3cc/0x7e0 [sunrpc ae3a15912ae37fd51dafbdbc2dbd069117f8f5c8]
[  224.156367]  ? xs_tcp_tls_setup_socket+0x2fe/0x7e0 [sunrpc ae3a15912ae37fd51dafbdbc2dbd069117f8f5c8]
[  224.156697]  ? __pfx_xs_tls_handshake_done+0x10/0x10 [sunrpc ae3a15912ae37fd51dafbdbc2dbd069117f8f5c8]
[  224.157013]  process_scheduled_works+0x24e/0x450
[  224.157158]  worker_thread+0x21c/0x2d0
[  224.157275]  ? __pfx_worker_thread+0x10/0x10
[  224.157409]  kthread+0xe8/0x110
[  224.157510]  ? __pfx_kthread+0x10/0x10
[  224.157628]  ret_from_fork+0x37/0x50
[  224.157741]  ? __pfx_kthread+0x10/0x10
[  224.157859]  ret_from_fork_asm+0x1b/0x30
[  224.157983]  </TASK>

Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>