SUNRPC: Clean up xdr_get_next_encode_buffer()

The value of @p is not used until the "location of the next item" is
computed. Help human readers by moving its initial assignment to the
paragraph where that value is used and by clarifying the antecedents
in the documenting comment.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: NeilBrown <neilb@suse.com>
Reviewed-by: J. Bruce Fields <bfields@fieldses.org>