rxrpc: fix some null-ptr-deref bugs in server_key.c

Some function calls are not implemented in rxrpc_no_security, there are
preparse_server_key, free_preparse_server_key and destroy_server_key.
When rxrpc security type is rxrpc_no_security, user can easily trigger a
null-ptr-deref bug via ioctl. So judgment should be added to prevent it

The crash log:
user@syzkaller:~$ ./rxrpc_preparse_s
[   37.956878][T15626] BUG: kernel NULL pointer dereference, address: 0000000000000000
[   37.957645][T15626] #PF: supervisor instruction fetch in kernel mode
[   37.958229][T15626] #PF: error_code(0x0010) - not-present page
[   37.958762][T15626] PGD 4aadf067 P4D 4aadf067 PUD 4aade067 PMD 0
[   37.959321][T15626] Oops: 0010 [#1] PREEMPT SMP
[   37.959739][T15626] CPU: 0 PID: 15626 Comm: rxrpc_preparse_ Not tainted 5.17.0-01442-gb47d5a4f6b8d #43
[   37.960588][T15626] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1 04/01/2014
[   37.961474][T15626] RIP: 0010:0x0
[   37.961787][T15626] Code: Unable to access opcode bytes at RIP 0xffffffffffffffd6.
[   37.962480][T15626] RSP: 0018:ffffc9000d9abdc0 EFLAGS: 00010286
[   37.963018][T15626] RAX: ffffffff84335200 RBX: ffff888012a1ce80 RCX: 0000000000000000
[   37.963727][T15626] RDX: 0000000000000000 RSI: ffffffff84a736dc RDI: ffffc9000d9abe48
[   37.964425][T15626] RBP: ffffc9000d9abe48 R08: 0000000000000000 R09: 0000000000000002
[   37.965118][T15626] R10: 000000000000000a R11: f000000000000000 R12: ffff888013145680
[   37.965836][T15626] R13: 0000000000000000 R14: ffffffffffffffec R15: ffff8880432aba80
[   37.966441][T15626] FS:  00007f2177907700(0000) GS:ffff88803ec00000(0000) knlGS:0000000000000000
[   37.966979][T15626] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   37.967384][T15626] CR2: ffffffffffffffd6 CR3: 000000004aaf1000 CR4: 00000000000006f0
[   37.967864][T15626] Call Trace:
[   37.968062][T15626]  <TASK>
[   37.968240][T15626]  rxrpc_preparse_s+0x59/0x90
[   37.968541][T15626]  key_create_or_update+0x174/0x510
[   37.968863][T15626]  __x64_sys_add_key+0x139/0x1d0
[   37.969165][T15626]  do_syscall_64+0x35/0xb0
[   37.969451][T15626]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[   37.969824][T15626] RIP: 0033:0x43a1f9

Signed-off-by: Xiaolong Huang <butterflyhuangxx@gmail.com>
Tested-by: Xiaolong Huang <butterflyhuangxx@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Link: http://lists.infradead.org/pipermail/linux-afs/2022-March/005069.html
Fixes: 12da59fcab5a ("rxrpc: Hand server key parsing off to the security class")
Link: https://lore.kernel.org/r/164865013439.2941502.8966285221215590921.stgit@warthog.procyon.org.uk
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
