NFS: introduce generic decode_getattr function

Getattr should be able to decode errors and the readdir file handle.
decode_getfattr_attrs does the actual attribute decoding, while
decode_getfattr_generic will check the opcode before decoding.  This will
let other functions call decode_getfattr_attrs to decode their attributes.

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>