samples: net: fix a memcmp len error

The memcmp is a comparison between two strings or buffers.
So, the length should be the buffer length, not the length
of the pointer to the buffer. This was caught by LLVM.

Jira: ZEP-1179

Change-Id: I7fd6b199686b19e7f4a2e1288897483e69ad091e
Signed-off-by: Baohong Liu <baohong.liu@intel.com>