[XFS] split out two helpers from xfs_syncsub

Split out two helpers from xfs_syncsub for the dummy log commit and the
superblock writeout.

SGI-PV: 988140

SGI-Modid: xfs-linux-melb:xfs-kern:32303a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: David Chinner <david@fromorbit.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>