Bluetooth: Silence DEBUG_STRICT_USER_COPY_CHECKS=y warning

Bluetooth: Silence DEBUG_STRICT_USER_COPY_CHECKS=y warning

Enabling DEBUG_STRICT_USER_COPY_CHECKS causes the following
warning:

In function 'copy_from_user',
    inlined from 'rfcomm_sock_setsockopt' at
    net/bluetooth/rfcomm/sock.c:705:
arch/x86/include/asm/uaccess_64.h:65:
warning: call to 'copy_from_user_overflow' declared with
attribute warning: copy_from_user() buffer size is not provably
correct

presumably due to buf_size being signed causing GCC to fail to
see that buf_size can't become negative.

Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: Gustavo F. Padovan <padovan@profusion.mobi>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>