MINOR: fd: centralize poll timeout computation in compute_poll_timeout()

MINOR: fd: centralize poll timeout computation in compute_poll_timeout()

The 4 pollers all contain the same code used to compute the poll timeout.
This is pointless, let's centralize this into fd.h. This also gets rid of
the useless SCHEDULER_RESOLUTION macro which used to work arond a very old
linux 2.2 bug causing select() to wake up slightly before the timeout.