NFS: Fix a bogus warning in nfs_generic_pgio

NFS: Fix a bogus warning in nfs_generic_pgio

commit b8fb9c30f25e45dab5d2cd310ab6913b6861d00f upstream.

It is OK for pageused == pagecount in the loop, as long as we don't add
another entry to the *pages array. Move the test so that it only triggers
in that case.

Reported-by: Steve Dickson <SteveD@redhat.com>
Fixes: bba5c1887a92 (nfs: disallow duplicate pages in pgio page vectors)
Cc: Weston Andros Adamson <dros@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>