avformat/flvenc: Avoid unnecessary seek

avformat/flvenc: Avoid unnecessary seek

When shifting the already written data in order to write the keyframe
index, the flv muxer would first store the pre-shift size, then
calculate how big the index will be eventually, then perform some seeks
to update some size fields, then seek back to the end of the file to get
the new position, followed by a seek to the position where writing will
really start. Seeking back to the (already known) end position (that is
actually used to perform this seek) to get the end position is of course
unnecessary. It has been removed.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>