Cleanup: lib ring buffer: rename pagecpy local variable

Cleanup: lib ring buffer: rename pagecpy local variable

Rename pagecpy local variable to bytes_left_in_page, which has a clearer
semantic. Also remove the unused pagecpy parameter from slow paths which
was always 0.

Also add a missing __user annotation on @src of
the _lib_ring_buffer_copy_from_user_inatomic prototype.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I22a85cb8e94a65aaa80ca49951479243477ac600