mount: support idmapped mount points

This patch introduces idmapped mounts support for
container rootfs.

The idmapped mounts support was merged in Linux kernel 5.12
torvalds/linux@7d6beb7.
This functionality allows to address chown overhead for containers that
use user namespace.

The changes are based on experimental patchset published by
Mauricio Vásquez #4734.
Current version reiplements support of idmapped mounts using Golang.

Performance measurement results:
Image           idmapped mount  recursive chown
BusyBox         00.135          04.964
Ubuntu          00.171          15.713
Fedora          00.143          38.799

Signed-off-by: Mauricio Vásquez <mauricio@kinvolk.io>
Signed-off-by: Artem Kuzin <artem.kuzin@huawei.com>
Signed-off-by: Alexey Perevalov <alexey.perevalov@huawei.com>
Signed-off-by: Ilya Hanov <ilya.hanov@huawei-partners.com>

sys/subprocess_unsafe_linux.go

@@ -0,0 +1,30 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package sys
+
+import (
+	_ "unsafe" // required for go:linkname.
+)
+
+//go:linkname beforeFork syscall.runtime_BeforeFork
+func beforeFork()
+
+//go:linkname afterFork syscall.runtime_AfterFork
+func afterFork()
+
+//go:linkname afterForkInChild syscall.runtime_AfterForkInChild
+func afterForkInChild()