Bluetooth: RFCOMM: convert to getsockopt_iter

Convert RFCOMM socket's getsockopt implementation to use the new
getsockopt_iter callback with sockopt_t.

Key changes:
- Replace (char __user *optval, int __user *optlen) with sockopt_t *sopt
- Use sopt->optlen for buffer length (input)
- Use copy_to_iter() instead of put_user()/copy_to_user()
- Add linux/uio.h for copy_to_iter()

The sockopt_t parameter is named sopt rather than opt to avoid
collision with the existing local u32 opt used by RFCOMM_LM.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>