i2c: Push ioctl BKL down into the i2c code

This is part of the effort to get rid of the BKL.

[JD: In fact i2c-dev doesn't need more locking than is already done
for the other i2c drivers, so we can simply switch to unlocked_ioctl.]

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>