libceph: validate banner payload length

In the Linux kernel, the following vulnerability has been resolved:

libceph: validate banner payload length

When parsing the Ceph messenger v2 protocol banner, the `payload_len` field
is decoded from the banner prefix. If a client sends a banner with a
`payload_len` of 0, the kernel sets up a 0-length socket read. This
violates an invariant in the state machine, triggering a warning in
`populate_in_iter()`:

------------[ cut here ]------------
!iov_iter_count(&con->v2.in_iter)
WARNING: net/ceph/messenger_v2.c:3129 at populate_in_iter
net/ceph/messenger_v2.c:3129 [inline], CPU#1: kworker/1:3/5070
WARNING: net/ceph/messenger_v2.c:3129 at ceph_con_v2_try_read+0x6634/0x6810
net/ceph/messenger_v2.c:3159, CPU#1: kworker/1:3/5070
...
Call Trace:
<TASK>
ceph_con_workfn+0x1f5/0x14a0 net/ceph/messenger.c:1575
process_one_work kernel/workqueue.c:3322 [inline]
process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3405
worker_thread+0xa47/0xfb0 kernel/workqueue.c:3486
kthread+0x388/0x470 kernel/kthread.c:436
ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>

According to the msgr2 protocol specification, the banner payload is
expected to contain at least two 64-bit integers (`server_feat` and
`server_req_feat`). Therefore, `payload_len` must be at least 16 bytes.

Fix this by adding a check in `process_banner_prefix()` to reject a
`payload_len` smaller than 16 bytes. This prevents the 0-length read and
correctly aborts the connection with a protocol error.