[XFS] vnode cleanup in xfs_fs_subr.c

Cleanup the unneeded intermediate vnode step in the flushing helpers and
go directly from the xfs_inode to the struct address_space.

SGI-PV: 976035
SGI-Modid: xfs-linux-melb:xfs-kern:30530a

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