SUNRPC: Fix xdr_encode_bool()

I discovered that xdr_encode_bool() was returning the same address
that was passed in the @p parameter. The documenting comment states
that the intent is to return the address of the next buffer
location, just like the other "xdr_encode_*" helpers.

The result was the encoded results of NFSv3 PATHCONF operations were
not formed correctly.

Fixes: ded04a58 ("NFSD: Update the NFSv3 PATHCONF3res encoder to use struct xdr_stream")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>