rpc: move process_xdr_buf

Since process_xdr_buf() is useful outside of the kerberos-specific code, we
move it to net/sunrpc/xdr.c, export it, and rename it in keeping with xdr_*
naming convention of xdr.c.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>