Legends game periodically (every 30 seconds) calls this function
with up to 22k virtual addresses. All but 1 of them is valid. Due to
amount of queries addresses, and cost of seek+read, this causes this
function to take up to about 50ms. So framerate drops from ~150 FPS to
20FPS for about a second.

As far as I can see, returning 0 entries from this function, still makes
Apex Legend work.

But keep code correct, and optimise it by only performing pagemap read
when the address is valid.

This change get_working_set_ex reduces peek wall clock runtime from 57ms
to 0.29ms.

Tested on Linux, but similar change done for the BSD part.
Signed-off-by: Witold Baryluk <witold.baryluk@gmail.com>