powerpc/interrupts: Fix kuep_unlock() call

Same as kuap_user_restore(), kuep_unlock() has to be called when
really returning to user, that is in interrupt_exit_user_prepare(),
not in interrupt_exit_prepare().

Fixes: b5efec00 ("powerpc/32s: Move KUEP locking/unlocking in C")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/b831e54a2579db24fbef836ed415588ce2b3e825.1620312573.git.christophe.leroy@csgroup.eu