SUNRPC: use max_t() to simplify open code

Use max_t() to simplify open code which uses "if...else" to get maximum of
two values.

Generated by coccinelle script:
	scripts/coccinelle/misc/minmax.cocci

Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>