app/testpmd: fix DDP package filesize detection

app/testpmd: fix DDP package filesize detection

This issue was about passing unsigned argument where should be signed
number.
In reality this is about wrong usage of fseek and ftell to determine
filesize.
This patch is compliant to suggestions from FIO19-C:
"Do not use fseek() and ftell() to compute the size of a regular file"

Coverity issue: 143454
Fixes: a92a5a2cbbff ("app/testpmd: add command for loading DDP")
Cc: stable@dpdk.org
Signed-off-by: Daniel Mrzyglod <danielx.t.mrzyglod@intel.com>
Acked-by: Beilei Xing <beilei.xing@intel.com>