nfs: Remove unused xdr page offsets in getacl/setacl arguments

The arguments passed around for getacl and setacl xdr encoding, struct
nfs_setaclargs and struct nfs_getaclargs, both contain an array of
pages, an offset into the first page, and the length of the page data.
The offset is unused as it is always zero; remove it.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>