NFS: simplify inlined bit ops in nfs_page.h

 Minor cleanup:  inlined bit ops in nfs_page.h can be simpler.

 Test plan:
 Write-intensive workload against a server that requires COMMITs.

Signed-off-by: Chuck Lever <cel@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>