powerpc/pkeys: Detach execute_only key on !PROT_EXEC

Disassociate the exec_key from a VMA if the VMA permission is not
PROT_EXEC anymore. Otherwise the exec_only key continues to be
associated with the vma, causing unexpected behavior.

The problem was reported on x86 by Shakeel Butt, which is also
applicable on powerpc.

Fixes: 5586cf61

 ("powerpc: introduce execute-only pkey")
Cc: stable@vger.kernel.org # v4.16+
Reported-by: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Ram Pai <linuxram@us.ibm.com>
Reviewed-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>