powerpc: Remove MSR_PR check in interrupt_exit_{user/kernel}_prepare()

In those hot functions that are called at every interrupt, any saved
cycle is worth it.

interrupt_exit_user_prepare() and interrupt_exit_kernel_prepare() are
called from three places:
- From entry_32.S
- From interrupt_64.S
- From interrupt_exit_user_restart() and interrupt_exit_kernel_restart()

In entry_32.S, there are inambiguously called based on MSR_PR:

	interrupt_return:
		lwz	r4,_MSR(r1)
		addi	r3,r1,STACK_FRAME_OVERHEAD
		andi.	r0,r4,MSR_PR
		beq	.Lkernel_interrupt_return
		bl	interrupt_exit_user_prepare
	...
	.Lkernel_interrupt_return:
		bl	interrupt_exit_kernel_prepare

In interrupt_64.S, that's similar:

	interrupt_return_\srr\():
		ld	r4,_MSR(r1)
		andi.	r0,r4,MSR_PR
		beq	interrupt_return_\srr\()_kernel
	interrupt_return_\srr\()_user: /* make backtraces match the _kernel variant */
		addi	r3,r1,STACK_FRAME_OVERHEAD
		bl	interrupt_exit_user_prepare
	...
	interrupt_return_\srr\()_kernel:
		addi	r3,r1,STACK_FRAME_OVERHEAD
		bl	interrupt_exit_kernel_prepare

In interrupt_exit_user_restart() and interrupt_exit_kernel_restart(),
MSR_PR is verified respectively by BUG_ON(!user_mode(regs)) and
BUG_ON(user_mode(regs)) prior to calling interrupt_exit_user_prepare()
and interrupt_exit_kernel_prepare().

The verification in interrupt_exit_user_prepare() and
interrupt_exit_kernel_prepare() are therefore useless and can be removed.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/385ead49ccb66a259b25fee3eebf0bd4094068f3.1629707037.git.christophe.leroy@csgroup.eu

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