BUG/MEDIUM: fd: Don't modify the update_mask in fd_dodelete().

BUG/MEDIUM: fd: Don't modify the update_mask in fd_dodelete().

Only the pollers should remove bits in the update_mask. Removing it will
mean if the fd is currently in the global update list, it will never be
removed, and while it's mostly harmless in 1.9, in 1.8, only update_mask
is checked to know if the fd is already in the list or not, so we can end
up trying to add a fd that is already in the list, and corrupt it, which
means some fd may not be added to the poller.

This should be backported to 1.8.