xfs: merge xfs_inode_flush into xfs_fs_write_inode

Splitting the task for a VFS-induced inode flush into two functions doesn't
make any sense, so merge the two functions dealing with it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Felix Blyakher <felixb@sgi.com>
Reviewed-by: Dave Chinner <david@fromorbit.com>