GFS2: Re-add a call to log_flush_wait when flushing the journal

Upstream commit 34cc1781 changed a line of code from calling function
log_flush_commit to calling log_write_header. This had the effect of
eliminating a call to function log_flush_wait. That causes the journal
to skip over log headers, which results in multiple wrap points,
which itself leads to infinite loops in journal replay, both in the
kernel code and fsck.gfs2 code. This patch re-adds that call.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>