After a successful layout swap, the data version of a file is sometimes not changed.

llapi_get_data_version(fd1, &dv1, LL_DV_RD_FLUSH);  -> get data version
llapi_fswap_layouts(fd1, fd2, ....) -> success
lapi_get_data_version(fd1, llapi_get_data_version(fd1, &dv1, LL_DV_RD_FLUSH);  -> get same data version

Sometimes the dataversion is different, and sometimes it is not. So some caching might be involved.

I added some calls to sync() and fsync() on both file descriptors just after the call to layout swap, but that didn't fix the problem.

I pushed a reproducer in http://review.whamcloud.com/#/c/13441/. Change "#if 0" to "#if 1" in swap_layout_test.c:test42(). The calls to tests 30 and 31 should be commented out too since they are not related and take too long to execute.

	if (0) PERFORM(test30);
	if (0) PERFORM(test31);