mxfdemux: Fix seeking to the end of file

mxfdemux: Fix seeking to the end of file

When seeking to a position where no new offset can be
found or after the end of a track don't set the EOS flag
to TRUE as we don't (and can't) push EOS downstream then.
Instead set the current essence track position to then
end of the essence track and let the loop or chain function
set everything to EOS and push EOS downstream.

Also restart the pad's task if resolving metadata failed
(playback will continue as before) and return FALSE from
the seek if no new offset could be found.