powerpc/interrupt: Refactor prep_irq_for_{user/kernel_enabled}_exit()

prep_irq_for_user_exit() is a superset of
prep_irq_for_kernel_enabled_exit().

Rename prep_irq_for_kernel_enabled_exit() as prep_irq_for_enabled_exit()
and have prep_irq_for_user_exit() use it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210617155116.2167984-17-npiggin@gmail.com