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_sy…