checkpoint: fix running do_dump()

checkpoint: fix running do_dump()

Testing 'lxc <container> stop --stateful' crashed LXD:

fatal error: unexpected signal during runtime execution
[signal SIGSEGV: segmentation violation code=0x1 addr=0xe8 pc=0x7f3198ff0592]

runtime stack:
runtime.throw(0x117fe4a, 0x2a)
	/opt/rh/go-toolset-1.10/root/usr/lib/go-toolset-1.10-golang/src/runtime/panic.go:616 +0x81
runtime.sigpanic()
	/opt/rh/go-toolset-1.10/root/usr/lib/go-toolset-1.10-golang/src/runtime/signal_unix.go:372 +0x28e

goroutine 375 [syscall]:
runtime.cgocall(0xef38e3, 0xc420731630, 0x29)
	/opt/rh/go-toolset-1.10/root/usr/lib/go-toolset-1.10-golang/src/runtime/cgocall.go:128 +0x64 fp=0xc4207315f0 sp=0xc4207315b8 pc=0x410fc4
gopkg.in/lxc/go-lxc%2ev2._Cfunc_go_lxc_migrate(0x7f316c001220, 0xc400000001, 0xc420302460, 0xc4205d6080, 0x0)
	_cgo_gotypes.go:752 +0x4d fp=0xc420731630 sp=0xc4207315f0 pc=0x909d7d
gopkg.in/lxc/go-lxc%2ev2.(*Container).Migrate.func4(0x7f316c001220, 0xc400000001, 0xc420302460, 0xc4205d6080, 0x0)
	/share/go/src/gopkg.in/lxc/go-lxc.v2/container.go:1798 +0x160 fp=0xc420731668 sp=0xc420731630 pc=0x91b970
gopkg.in/lxc/go-lxc%2ev2.(*Container).Migrate(0xc4207a52f0, 0x1, 0xc42051ec00, 0x20, 0x0, 0x0, 0x0, 0x0, 0x101, 0x10000000, ...)
	/share/go/src/gopkg.in/lxc/go-lxc.v2/container.go:1798 +0x29f fp=0xc420731760 sp=0xc420731668 pc=0x9160ef

The commit 5a087e056f94 introduced a second parameter (conf) to the
cgroup escape() function which was never set in do_dump(). Instead of
taking it from opts->handler->conf it is now used from c->lxc_conf.

Fixes: 5a087e056f94 ("cgroups: don't escape if lxc.cgroup.keep is true")
Suggested-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Adrian Reber <areber@redhat.com>