move sys.FMountat() into mount package

It's the only location this is used, so might as well move it
into that package. I could not find external users of this utility,
so not adding an alias / deprecation.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

sys/subprocess_unsafe_linux.go

@@ -1,30 +0,0 @@
-/*
-   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()