BUG/MINOR: poll: fix abnormally high skip_fd counter

BUG/MINOR: poll: fix abnormally high skip_fd counter

The skip_fd counter that is incremented when a migrated FD is reported
was abnormally high in with poll. The reason is that it was accounted
for before preparing the polled events instead of being measured from
the reported events.

This mistake was done when the counters were introduced in 1.9 with
commit d80cb4ee1 ("MINOR: global: add some global activity counters to
help debugging"). It may be backported as far as 2.0.