cgroup: use restart_syscall() for mount retries

cgroup_mount() uses dumb delay-and-retry logic to wait for cgroup_root
which is being destroyed.  The retry currently loops inside
cgroup_mount() proper.  This patch makes it return with
restart_syscall() instead so that retry travels out to userland
boundary.

This slightly simplifies the logic and more importantly makes the
retry logic behave better when the wait for some reason becomes
lengthy or infinite by allowing the operation to be suspended or
terminated from userland.

v2: The original patch forgot to free memory allocated for @opts.
    Fixed.  Caught by Li Zefan.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>