arm64: set UXN on swapper page tables

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

arm64: set UXN on swapper page tables

[ This issue was fixed upstream by accident in c3cee924bd85 ("arm64:

head: cover entire kernel image in initial ID map") as part of a

large refactoring of the arm64 boot flow. This simple fix is therefore

preferred for -stable backporting ]

On a system that implements FEAT_EPAN, read/write access to the idmap

is denied because UXN is not set on the swapper PTEs. As a result,

idmap_kpti_install_ng_mappings panics the kernel when accessing

__idmap_kpti_flag. Fix it by setting UXN on these PTEs.