FileRequest: Make Stop() cancel the request as soon as possible

This commit implements a check for the flag raised by Stop() to cancel
a given request as soon as possible. Cancelled requests will return
B_INTERRUPTED regardless of whether the request has completed,
on par with how BHttpRequest is behaving at the moment.

Change-Id: Ia8a95b910cff158c710c5b2ed58b4675e705642e
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3071
Reviewed-by: waddlesplash <waddlesplash@gmail.com>