selftests: net: really check for bg process completion

A recent refactor transformed the check for process completion
in a true statement, due to a typo.

As a result, the relevant test-case is unable to catch the
regression it was supposed to detect.

Restore the correct condition.

Fixes: 691bb4e ("selftests: net: avoid just another constant wait")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/0e6f213811f8e93a235307e683af8225cc6277ae.1730828007.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Paolo Abeni

tools/testing/selftests/net/pmtu.sh

@@ -2056,7 +2056,7 @@ check_running() {
 	pid=${1}
 	cmd=${2}
 
-	[ "$(cat /proc/${pid}/cmdline 2>/dev/null | tr -d '\0')" = "{cmd}" ]
+	[ "$(cat /proc/${pid}/cmdline 2>/dev/null | tr -d '\0')" = "${cmd}" ]
 }
 
 test_cleanup_vxlanX_exception() {