This fixes the issue where "childFD" from syscall.Openat is assigned to
a local variable inside the for loop, instead of the correct one in the
function scope. This results in that when trying to close the "childFD"
in the function scope, it will be equal to "-1", instead of the correct
value.
Files in pkg/util/mount that contain significant code implementation for
subpaths are moved to a new package at pkg/volume/util/subpath. This
move is done in order to preserve git history.