zdtm: exclude vdso before modifing output of ldd

ldd pipe00:
linux-vdso.so.1 =>  (0x00007fffbb0cb000)

Currently the first part of line is dropts, if the line contains "=>",
so we need to exclude vdso, before modifing lines.

This patch fixes bug:
$ zdtm.sh ns/static/pipe00
Failed at  (0x00007fffa49fe000)

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>