selinux: preserve user SID across nested backing files

In the Linux kernel, the following vulnerability has been resolved:

selinux: preserve user SID across nested backing files

SELinux saves the user file SID in a backing-file security blob so it
remains available after mmap() replaces vma->vm_file with a backing file.

For nested backing files (overlayfs over overlayfs, or FUSE passthrough
backed by overlayfs), user_file may itself be a backing file. Its
fsec->sid is the SID of the mounter that opened it, rather than the user
that opened the top-level file. mprotect() then checks fd { use } against
the mounter SID. This can incorrectly deny access without a domain
transition, or check the wrong target SID after one.

Copy the saved user SID when user_file is a backing file. Keep using the
regular file SID for the first backing layer.

With two nested overlayfs mounts and SELinux enforcing,
mprotect(PROT_READ) returns EACCES with an fd { use } denial against the
mounter SID. With this change, mprotect() succeeds.

Tested on arm64 QEMU with a small BusyBox initramfs and a purpose-built
SELinux policy. The original test was also repeated with Fedora Cloud
Base 44 userspace and gave the same result.