powerpc/4xx: Fix setup_kuep() on SMP

On SMP, setup_kuep() is also called from start_secondary() since
commit 86f46f34 ("powerpc/32s: Initialise KUAP and KUEP in C").

start_secondary() is not an __init function.

Remove the __init marker from setup_kuep() and bail out when
not caller on the first CPU as the work is already done.

Fixes: 10248dcb ("powerpc/44x: Implement Kernel Userspace Exec Protection (KUEP)")
Fixes: 86f46f34

 ("powerpc/32s: Initialise KUAP and KUEP in C")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/8ee05934288994a65743a987acb1558f12c0c8c1.1624969450.git.christophe.leroy@csgroup.eu