arm64: Ensure execute-only permissions are not allowed without EPAN

Commit 18107f8a ("arm64: Support execute-only permissions with
Enhanced PAN") re-introduced execute-only permissions when EPAN is
available. When EPAN is not available, arch_filter_pgprot() is supposed
to change a PAGE_EXECONLY permission into PAGE_READONLY_EXEC. However,
if BTI or MTE are present, such check does not detect the execute-only
pgprot in the presence of PTE_GP (BTI) or MT_NORMAL_TAGGED (MTE),
allowing the user to request PROT_EXEC with PROT_BTI or PROT_MTE.

Remove the arch_filter_pgprot() function, change the default VM_EXEC
permissions to PAGE_READONLY_EXEC and update the protection_map[] array
at core_initcall() if EPAN is detected.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Fixes: 18107f8a ("arm64: Support execute-only permissions with Enhanced PAN")
Cc: <stable@vger.kernel.org> # 5.13.x
Acked-by: Will Deacon <will@kernel.org>
Reviewed-by: Vladimir Murzin <vladimir.murzin@arm.com>
Tested-by: Vladimir Murzin <vladimir.murzin@arm.com>