Bluetooth: Serialize SMP remote OOB data access

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

Bluetooth: Serialize SMP remote OOB data access

build_pairing_cmd() looks up remote OOB data and copies its contents
without holding hdev->lock, which serializes the list's writers. After
SMP finds an entry, a concurrent management Remove Remote OOB Data
command can unlink and free it before SMP reads its present flag or
copies its random and confirmation values. Removal can also invalidate
an entry while the lookup is still traversing the list.

KASAN reported:

BUG: KASAN: slab-use-after-free in build_pairing_cmd+0x948/0x9b0
Call Trace:
build_pairing_cmd+0x948/0x9b0
smp_recv_cb+0x459f/0x8110
l2cap_recv_frame+0xf14/0x9190
l2cap_recv_acldata+0xa64/0xd40
hci_rx_work+0x4ca/0x730

Allocated by task 87:
hci_add_remote_oob_data+0x11d/0x530
add_remote_oob_data+0x282/0x400
hci_sock_sendmsg+0x1033/0x1ea0

Freed by task 93:
hci_remote_oob_data_clear+0x108/0x1c0
remove_remote_oob_data+0x198/0x220
hci_sock_sendmsg+0x1033/0x1ea0

Taking hdev->lock in build_pairing_cmd() would recurse for callers that
already hold it and invert the device-to-L2CAP lock order on the receive
path. Add a per-device remote_oob_lock instead, held across the SMP
lookup and copies and by the add, remove and clear helpers. Cover
initialization and in-place updates as well, so SMP cannot read partially
initialized or updated OOB values. Release the mutex on allocation
failure, preserving the existing error return.

The new critical sections acquire no device, connection or channel locks.
Writers retain their existing hdev->lock protection, which continues to
serialize the other readers without changing their locking or behavior.