gfs2: Remove active journal side effect from gfs2_write_log_header

Function gfs2_write_log_header can be used to write a log header into any of
the journals of a filesystem.  When used on the node's own journal,
gfs2_write_log_header advances the current position in the log
(sdp->sd_log_flush_head) as a side effect, through function gfs2_log_bmap.

This is confusing, and it also means that we can't use gfs2_log_bmap for other
journals even if they have an extent map.  So clean this mess up by not
advancing sdp->sd_log_flush_head in gfs2_write_log_header or gfs2_log_bmap
anymore and making that a responsibility of the callers instead.

This is related to commit 7c70b896

 ("gfs2: clean_journal improperly set
sd_log_flush_head").

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>