NFS: move nfs_pgarray_set() to open code

Since commit 00bfa30a ("NFS: Create a common pgio_alloc and
pgio_release function"), nfs_pgarray_set() has only a single caller.  Let's
open code it.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>