selftests: mm: make protection_keys test work on arm64

The encoding of the pkey register differs on arm64, than on x86/ppc. On those
platforms, a bit in the register is used to disable permissions, for arm64, a
bit enabled in the register indicates that the permission is allowed.

This drops two asserts of the form:
	 assert(read_pkey_reg() <= orig_pkey_reg);
Because on arm64 this doesn't hold, due to the encoding.

The pkey must be reset to both access allow and write allow in the signal
handler. pkey_access_allow() works currently for PowerPC as the
PKEY_DISABLE_ACCESS and PKEY_DISABLE_WRITE have overlapping bits set.

Access to the uc_mcontext is abstracted, as arm64 has a different structure.

Signed-off-by: Joey Gouly <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Acked-by: Dave Hansen <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>

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