gfs2: Don't get stuck with I/O plugged in gfs2_ail1_flush

In gfs2_ail1_flush, we're using I/O plugging to give the block layer a
better chance of merging I/O requests.  If we're too aggressive here, we
can end up waiting on I/O to complete while still plugged.  Fix that in
a way similar to writeback_sb_inodes, except that we can't use
blk_flush_plug because blk_flush_plug_list is not exported.

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