Bluetooth: iso: Fix circular lock in iso_conn_big_sync

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

Bluetooth: iso: Fix circular lock in iso_conn_big_sync

This fixes the circular locking dependency warning below, by reworking
iso_sock_recvmsg, to ensure that the socket lock is always released
before calling a function that locks hdev.

[ 561.670344] ======================================================
[ 561.670346] WARNING: possible circular locking dependency detected
[ 561.670349] 6.12.0-rc6+ #26 Not tainted
[ 561.670351] ------------------------------------------------------
[ 561.670353] iso-tester/3289 is trying to acquire lock:
[ 561.670355] ffff88811f600078 (&hdev->lock){+.+.}-{3:3},
at: iso_conn_big_sync+0x73/0x260 [bluetooth]
[ 561.670405]
but task is already holding lock:
[ 561.670407] ffff88815af58258 (sk_lock-AF_BLUETOOTH){+.+.}-{0:0},
at: iso_sock_recvmsg+0xbf/0x500 [bluetooth]
[ 561.670450]
which lock already depends on the new lock.

[ 561.670452]
the existing dependency chain (in reverse order) is:
[ 561.670453]
-> #2 (sk_lock-AF_BLUETOOTH){+.+.}-{0:0}:
[ 561.670458] lock_acquire+0x7c/0xc0
[ 561.670463] lock_sock_nested+0x3b/0xf0
[ 561.670467] bt_accept_dequeue+0x1a5/0x4d0 [bluetooth]
[ 561.670510] iso_sock_accept+0x271/0x830 [bluetooth]
[ 561.670547] do_accept+0x3dd/0x610
[ 561.670550] __sys_accept4+0xd8/0x170
[ 561.670553] __x64_sys_accept+0x74/0xc0
[ 561.670556] x64_sys_call+0x17d6/0x25f0
[ 561.670559] do_syscall_64+0x87/0x150
[ 561.670563] entry_SYSCALL_64_after_hwframe+0x76/0x7e
[ 561.670567]
-> #1 (sk_lock-AF_BLUETOOTH-BTPROTO_ISO){+.+.}-{0:0}:
[ 561.670571] lock_acquire+0x7c/0xc0
[ 561.670574] lock_sock_nested+0x3b/0xf0
[ 561.670577] iso_sock_listen+0x2de/0xf30 [bluetooth]
[ 561.670617] __sys_listen_socket+0xef/0x130
[ 561.670620] __x64_sys_listen+0xe1/0x190
[ 561.670623] x64_sys_call+0x2517/0x25f0
[ 561.670626] do_syscall_64+0x87/0x150
[ 561.670629] entry_SYSCALL_64_after_hwframe+0x76/0x7e
[ 561.670632]
-> #0 (&hdev->lock){+.+.}-{3:3}:
[ 561.670636] __lock_acquire+0x32ad/0x6ab0
[ 561.670639] lock_acquire.part.0+0x118/0x360
[ 561.670642] lock_acquire+0x7c/0xc0
[ 561.670644] __mutex_lock+0x18d/0x12f0
[ 561.670647] mutex_lock_nested+0x1b/0x30
[ 561.670651] iso_conn_big_sync+0x73/0x260 [bluetooth]
[ 561.670687] iso_sock_recvmsg+0x3e9/0x500 [bluetooth]
[ 561.670722] sock_recvmsg+0x1d5/0x240
[ 561.670725] sock_read_iter+0x27d/0x470
[ 561.670727] vfs_read+0x9a0/0xd30
[ 561.670731] ksys_read+0x1a8/0x250
[ 561.670733] __x64_sys_read+0x72/0xc0
[ 561.670736] x64_sys_call+0x1b12/0x25f0
[ 561.670738] do_syscall_64+0x87/0x150
[ 561.670741] entry_SYSCALL_64_after_hwframe+0x76/0x7e
[ 561.670744]
other info that might help us debug this:

[ 561.670745] Chain exists of:
&hdev->lock --> sk_lock-AF_BLUETOOTH-BTPROTO_ISO --> sk_lock-AF_BLUETOOTH

[ 561.670751] Possible unsafe locking scenario:

[ 561.670753] CPU0 CPU1
[ 561.670754] ---- ----
[ 561.670756] lock(sk_lock-AF_BLUETOOTH);
[ 561.670758] lock(sk_lock
AF_BLUETOOTH-BTPROTO_ISO);
[ 561.670761] lock(sk_lock-AF_BLUETOOTH);
[ 561.670764] lock(&hdev->lock);
[ 561.670767]
*** DEADLOCK ***