iio: core: Make mlock internal to the iio core

"mlock" should ideally only be used by the iio core. The mlock
implementation may change in the future which means that no driver
should be explicitly using mlock.

Signed-off-by: Rohit Sarkar <rohitsarkar5398@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>