direct I/O locking changes it is just a wrapper around
VOP_FLUSHINVAL_PAGES, so it's not nessecary anymore.  Keep a simplified
version for kernels < 2.4.22, as these don't have the changed direct I/O
locking.

SGI-PV: 938064
SGI-Modid: xfs-linux:xfs-kern:194420a
Signed-off-by: Christoph Hellwig <hch@sgi.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>