There are situations where the underlying filesystem is unreachable
(e.g: NFS) causing symlink resolution to hang. Such a situation
should be avoided by using umount -f -R (force and raw), but while -R
causes the symlink resolution to be skipped in umount(8), the kernel was
still doing it in sys_unmount(). This changes fixes that.
When the -R flag is not given, umount(8) does symlinks resolution through
realpath(3) before calling unmount(2), hence not doing it in the kernel
would not change behavior.