Freely copying from gtp-load-gen.c, implement GTP flooding in
osmo-pfcp-tool scripts.
Add dependency liburing. It can be disabled with
configure --disable-uring
in which case the new 'gtp flood' command only logs an error.
This verbosely commented example script serves as a good explanation:
contrib/osmo-pfcp-tool-scripts/gtp_flood.vty
Related: SYS#6590
Change-Id: I332aa0e2efd55f6e357cde4752a3d8b584db531b
