Add xdr_len, to work out required buffer size

Add xdr_len, to work out required buffer size

Currently, code which uses xdr_mem needs to take a guess at the
buffer size required, allocate that guess, and error out if the
data being encoded actually exceeds the guess.

This adds a new XDR mechanism - xdr_len, which can return
(using xdr_getpos) the length of the buffer required to XDR
encode a given structure.

Change-Id: I6e04c051118a26441caebdbcb2b5768d3c4389b6
Reviewed-on: http://gerrit.openafs.org/1221
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>