gfs2: Simplify gfs2_log_pointers_init

Move the initialization of sdp->sd_log_sequence and
sdp->sd_log_flush_head inside gfs2_log_pointers_init().  Use
gfs2_replay_incr_blk().

Before this change, the log head lookup code in freeze_go_xmote_bh()
didn't update sdp->sd_log_flush_head.  This is now fixed, but the code
in freeze_go_xmote_bh() appears to be pretty useless in the first place:
on a frozen filesystem, the log head will not change.

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