mount: strip options for all mounts

Currently we stript options only one of brothers, but
mount_equal() thinks that two brothers should have the same options.

Execute zdtm/live/static/mountpoints
./mountpoints --pidfile=mountpoints.pid --outfile=mountpoints.out
Dump 2737
WARNING: mountpoints returned 1 and left running for debug needs
Test: zdtm/live/static/mountpoints, Result: FAIL
==================================== ERROR ====================================
Test: zdtm/live/static/mountpoints, Namespace:
Dump log   : /root/git/criu/test/dump/static/mountpoints/2737/1/dump.log
--------------------------------- grep Error ---------------------------------
(00.146444) Error (mount.c:399): Two shared mounts 50, 67 have different sets of children
(00.146460) Error (mount.c:402): 67:./zdtm_mpts/dev/share-1 doesn't have a proper point for 54:./zdtm_mpts/dev/share-3/test.mnt.share
(00.146820) Error (cr-dump.c:1921): Dumping FAILED.
------------------------------------- END -------------------------------------
================================= ERROR OVER =================================

Reported-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Tested-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>