scripts: add bpf/readv.bt

This script captures a histogram of the returned values
from the readv syscall.  It can be useful for analyzing
the efficiency of our sock/posix implementation.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: I910eb2e81e3416855cda7a3854982cd02476850e
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/7712
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>

scripts/bpf/readv.bt

@@ -0,0 +1,3 @@
+tracepoint:syscalls:sys_exit_readv /pid == __PID__/ {
+	@bytes = hist(args->ret);
+}