SUNRPC: clean up integer overflow check

This integer overflow check works as intended but Clang and GCC and warn
about it when compiling with W=1.

    include/linux/sunrpc/xdr.h:539:17: error: comparison is always false
    due to limited range of data type [-Werror=type-limits]

Use size_mul() to prevent the integer overflow.  It silences the warning
and it's cleaner as well.

Reported-by: Dmitry Antipov <dmantipov@yandex.ru>
Closes: https://lore.kernel.org/all/20230601143332.255312-1-dmantipov@yandex.ru/


Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>