phonet/pep: disable BH around forwarded sk_receive_skb()

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

phonet/pep: disable BH around forwarded sk_receive_skb()

The networking receive path is usually run from softirq context, but
protocols that take the socket lock may have packets stored in the
backlog and processed later from process context. In that case
release_sock() -> __release_sock() drops the slock with spin_unlock_bh()
and then calls sk->sk_backlog_rcv() with bottom halves enabled.

Typical sk_backlog_rcv handlers process the socket whose backlog is
being drained, so the BH state at entry is irrelevant for the slocks
they touch. pep_do_rcv() is different: when the inbound skb targets an
existing PEP pipe, it forwards the skb to a different *child* socket
via sk_receive_skb(). That helper takes the child slock with
bh_lock_sock_nested(), which is just spin_lock_nested() and assumes BH
is already off. The same child slock therefore ends up acquired with
BH on (process path) and with BH off (softirq path):

process context softirq context
--------------- ---------------
release_sock(listener) __netif_receive_skb()
__release_sock() phonet_rcv()
spin_unlock_bh() __sk_receive_skb(listener)
[BH now ENABLED] [BH already disabled]
sk_backlog_rcv: sk_backlog_rcv:
pep_do_rcv() pep_do_rcv()
sk_receive_skb(child) sk_receive_skb(child)
bh_lock_sock_nested(child) bh_lock_sock_nested(child)
=> SOFTIRQ-ON-W => IN-SOFTIRQ-W

Lockdep flags this as inconsistent lock state, and it can become a real
self-deadlock if a softirq on the same CPU tries to receive to the same
child socket while its slock is held in the BH-enabled path:

WARNING: inconsistent lock state
inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
(slock-AF_PHONET/1){+.?.}-{3:3}, at: __sk_receive_skb+0x1cf/0x900
__sk_receive_skb net/core/sock.c:563
sk_receive_skb include/net/sock.h:2022 [inline]
pep_do_rcv net/phonet/pep.c:675
sk_backlog_rcv include/net/sock.h:1190
__release_sock net/core/sock.c:3216
release_sock net/core/sock.c:3815
pep_sock_accept net/phonet/pep.c:879

Wrap the forwarded sk_receive_skb() in local_bh_disable() /
local_bh_enable() so the child slock is always acquired with BH off.
local_bh_disable() nests safely on the softirq path.

Discovered via in-house syzkaller fuzzing; the same root cause also
on the linux-6.1.y syzbot dashboard as extid 44f0626dd6284f02663c.
Reproduced under KASAN + LOCKDEP + PROVE_LOCKING, reproducer:
https://pastebin.com/A3t8xzCR