image/dedup: use userns_call to be able to open images for write

When restoring a task in it's user namespace we had: "Unable to open
pages-3.img: Permission denied" when trying to open pages images for
write. (Write is needed for memory images deduplication feature to
work.)

These patch makes do_open_image open images fd through usernsd, to have
root permissions.

Also increase MAX_UNSFD_MSG_SIZE as path and metadata don't fit, and
remove a workaround in prepare_vma_ios.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>