Make dump_to_rootfs wait for 90s for real

When `failure_action` is set to `dump_to_rootfs`, the message:
"Waiting for rootfs mount, will timeout after 90 seconds"
is actually wrong. Kdump will simply call `systemctl start sysroot.mount`,
but the timeout value of sysroot.mount depends on the unit service and
dracut parameters. And by default, dracut will set
JobRunningTimeoutSec=0 and JobTimeoutSec=0 for the device units,
which means it will wait forever. (see wait_for_dev function in dracut)

For some devices, this can be fixed by setting rd.timeout=90. But when
initqueue is set enabled during initramfs build, dracut will force set
timeout for host devices to `0`. (see 99base/module-setup.sh).

Depending on dracut / systemd can make things unpredictable and break as
parameters or code change. To make things easy to understand and
maintain, just call `systemctl` with `--no-block` params, and implement
a standalone wait loop.  Now `dump_to_rootfs` will actually wait for
90s then timeout.

Signed-off-by: Kairui Song <kasong@redhat.com>
Acked-by: Coiby Xu <coxu@redhat.com>

kdump-lib-initramfs.sh

@@ -230,10 +230,20 @@ dump_to_rootfs()
     dinfo "Clean up dead systemd services"
     systemctl cancel
     dinfo "Waiting for rootfs mount, will timeout after 90 seconds"
-    systemctl start sysroot.mount
+    systemctl start --no-block sysroot.mount
+
+    _loop=0
+    while [ $_loop -lt 90 ] && ! is_mounted /sysroot; do
+        sleep 1
+        _loop=$((_loop + 1))
+    done
+
+    if ! is_mounted /sysroot; then
+        derror "Failed to mount rootfs"
+        return
+    fi
 
     ddebug "NEWROOT=$NEWROOT"
-
     dump_fs $NEWROOT
 }