WB_SYNC_NONE is supposed to mean "don't wait on anything". That should
also include not waiting for COMMIT calls to complete.

WB_SYNC_NONE is also implied when wbc->nonblocking and
wbc->for_background are set, so we can replace those checks in
nfs_commit_unstable_pages with a check for WB_SYNC_NONE.
Signed-off-by: NJeff Layton <jlayton@redhat.com>
Reviewed-by: NWu Fengguang <fengguang.wu@intel.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>