net: clear msg_get_inq in __sys_recvfrom() and __copy_msghdr_from_user()

syzbot reported uninit-value in tcp_recvmsg() [1]

Issue here is that msg->msg_get_inq should have been cleared,
otherwise tcp_recvmsg() might read garbage and perform
more work than needed, or have undefined behavior.

Given CONFIG_INIT_STACK_ALL_ZERO=y is probably going to be
the default soon, I chose to change __sys_recvfrom() to clear
all fields but msghdr.addr which might be not NULL.

For __copy_msghdr_from_user(), I added an explicit clear
of kmsg->msg_get_inq.

[1]
BUG: KMSAN: uninit-value in tcp_recvmsg+0x6cf/0xb60 net/ipv4/tcp.c:2557
tcp_recvmsg+0x6cf/0xb60 net/ipv4/tcp.c:2557
inet_recvmsg+0x13a/0x5a0 net/ipv4/af_inet.c:850
sock_recvmsg_nosec net/socket.c:995 [inline]
sock_recvmsg net/socket.c:1013 [inline]
__sys_recvfrom+0x696/0x900 net/socket.c:2176
__do_sys_recvfrom net/socket.c:2194 [inline]
__se_sys_recvfrom net/socket.c:2190 [inline]
__x64_sys_recvfrom+0x122/0x1c0 net/socket.c:2190
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x46/0xb0

Local variable msg created at:
__sys_recvfrom+0x81/0x900 net/socket.c:2154
__do_sys_recvfrom net/socket.c:2194 [inline]
__se_sys_recvfrom net/socket.c:2190 [inline]
__x64_sys_recvfrom+0x122/0x1c0 net/socket.c:2190

CPU: 0 PID: 3493 Comm: syz-executor170 Not tainted 5.19.0-rc3-syzkaller-30868-g4b28366af7d9 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011

Fixes: f94fd25c

 ("tcp: pass back data left in socket after receive")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Tested-by: Alexander <Potapenko&lt;glider@google.com>
Link: https://lore.kernel.org/r/20220622150220.1091182-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>