Bluetooth: Fix l2cap_sock_setsockopt() with optname BT_RCVMTU

commit 23bc6ab0a0912146fd674a0becc758c3162baabc upstream.

When we retrieve imtu value from userspace we should use 16 bit pointer
cast instead of 32 as it's defined that way in headers. Fixes setsockopt
calls on big-endian platforms.

Signed-off-by: Amadeusz Sławiński <amadeusz.slawinski@tieto.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Cc: stable@vger.kernel.org
Signed-off-by: Willy Tarreau <w@1wt.eu>