MINOR: poll: more accurately compute the new maxfd in the loop

MINOR: poll: more accurately compute the new maxfd in the loop

Last commit 173d995 ("MEDIUM: polling: start to move maxfd computation
to the pollers") moved the maxfd computation to the polling loop, but
it still adds an entry when removing an fd, forcing the next loop to
seek from further away than necessary. Let's only update the max when
actually adding an entry.