api: fix vl_api_clnt_node process stack overflow

Type: fix

Some simple settings(e.g. bringing up an i40e/ice interface) through
vnat consume more than the currently available stack space.

This root cause of this issue is same with commit
b2dbb36fc265b8996fc7fa310dda447d5b0479cb "vlib: fix startup-config \
-process stack overflow" and commit 2fd44a00aa26188ca75f0accd734f2 \
1758c199bf "vlib: fix cli process stack overflow"

Signed-off-by: Chenmin Sun <chenmin.sun@intel.com>
Change-Id: I312e4fed96a679aa68b859e28a90a2a4b6eb0c6e