arm64: io: Reject non-user protection in ioremap_prot()

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

arm64: io: Reject non-user protection in ioremap_prot()

Mapping a stack-top page via /dev/mem with PROT_NONE and then
reading that process's /proc/<pid>/cmdline triggers a spurious WARN
in ioremap_prot() through generic_access_phys():

WARNING: ./arch/arm64/include/asm/io.h:275 at generic_access_phys
Call trace:
generic_access_phys+0x1c8/0x228 (P)
__access_remote_vm+0x2b4/0x398
access_remote_vm+0x14/0x30
get_mm_cmdline+0xf8/0x2a0
proc_pid_cmdline_read+0x68/0x120

generic_access_phys() passes the protection derived from the user PTE
to ioremap_prot(). On arm64, a PROT_NONE mapping is represented by a
present-invalid PTE, so pte_present() still returns true and the
protection reaches ioremap_prot().

A PROT_NONE mapping does not have PTE_USER, causing the existing
WARN_ON_ONCE() in ioremap_prot() to fire even though this is a valid
user mapping. Execute-only mappings have the same issue and must not
be readable through this path either.

ioremap_prot() should therefore reject protection values without
PTE_USER without warning. This makes the access fail cleanly for
PROT_NONE and execute-only mappings while retaining the existing
user-protection contract.