NFSv4: Clean up pNFS return-on-close error handling

Both close and delegreturn have identical code to handle pNFS
return-on-close. This patch refactors that code and places it
in pnfs.c

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>