rds: tcp: fix uninit-value in __inet_bind

KMSAN reported an uninit-value access in __inet_bind() when binding
an RDS TCP socket.

The uninitialized memory originates from rds_tcp_conn_alloc(),
which uses kmem_cache_alloc() to allocate the rds_tcp_connection structure.

Specifically, the field 't_client_port_group' is incremented in
rds_tcp_conn_path_connect() without being initialized first:

if (++tc->t_client_port_group >= port_groups)

Since kmem_cache_alloc() does not zero the memory, this field contains
garbage, leading to the KMSAN report.

Fix this by using kmem_cache_zalloc() to ensure the structure is
zero-initialized upon allocation.

Reported-by: syzbot+aae646f09192f72a68dc@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=aae646f09192f72a68dc
Tested-by: syzbot+aae646f09192f72a68dc@syzkaller.appspotmail.com
Fixes: a20a6992558f ("net/rds: Encode cp_index in TCP source port")

Signed-off-by: Tabrez Ahmed <tabreztalks@gmail.com>
Reviewed-by: Charalampos Mitrodimas <charmitro@posteo.net>
Reviewed-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/20260217135350.33641-1-tabreztalks@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>