tipc: fix u16 MTU truncation in media and bearer MTU validation

tipc: fix u16 MTU truncation in media and bearer MTU validation

Both TIPC_NL_MEDIA_SET and TIPC_NL_BEARER_SET accept user-supplied
MTU values but only enforce a minimum bound, not a maximum. When a user
sets the MTU to a value exceeding U16_MAX (65535), it passes validation
but is silently truncated when assigned to u16 fields l->mtu and
l->advertised_mtu in tipc_link_create(). Values like 65536 (0x10000)
truncate to 0, causing a division by zero in tipc_link_set_queue_limits()
which computes TIPC_MAX_PUBL / (l->mtu / ITEM_SIZE). Other overflowing
values (e.g. 65537-131071) produce small incorrect MTU values, resulting
in link malfunction behaviors.

Crash stack (triggered as unprivileged user via user namespace):

  tipc_link_set_queue_limits  net/tipc/link.c:2531
  tipc_link_create            net/tipc/link.c:520
  tipc_node_check_dest        net/tipc/node.c:1279
  tipc_disc_rcv               net/tipc/discover.c:252
  tipc_rcv                    net/tipc/node.c:2129
  tipc_udp_recv               net/tipc/udp_media.c:392

Two independent paths lack the upper bound check:
1. tipc_udp_mtu_bad() -- called from __tipc_nl_media_set() (MEDIA_SET)
2. inline check in __tipc_nl_bearer_set() at bearer.c:1160 (BEARER_SET)

Fix both by rejecting MTU values above U16_MAX.

Fixes: 901271e0403a ("tipc: implement configuration of UDP media MTU")
Reported-by: AutonomousCodeSecurity@microsoft.com
Closes: https://lore.kernel.org/all/CAB8m9WgETt0AjmFwE=F-CKjGXsK6_WDv0=kbYRcC8-noo+amnA@mail.gmail.com
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Signed-off-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260714041541.307702-1-blbllhy@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>