powerpc/ftrace: Use patch_instruction() return directly

Instead of returning -EPERM when patch_instruction() fails,
just return what patch_instruction returns.

That simplifies ftrace_modify_code():

	   0:	94 21 ff c0 	stwu    r1,-64(r1)
	   4:	93 e1 00 3c 	stw     r31,60(r1)
	   8:	7c 7f 1b 79 	mr.     r31,r3
	   c:	40 80 00 30 	bge     3c <ftrace_modify_code+0x3c>
	  10:	93 c1 00 38 	stw     r30,56(r1)
	  14:	7c 9e 23 78 	mr      r30,r4
	  18:	7c a4 2b 78 	mr      r4,r5
	  1c:	80 bf 00 00 	lwz     r5,0(r31)
	  20:	7c 1e 28 40 	cmplw   r30,r5
	  24:	40 82 00 34 	bne     58 <ftrace_modify_code+0x58>
	  28:	83 c1 00 38 	lwz     r30,56(r1)
	  2c:	7f e3 fb 78 	mr      r3,r31
	  30:	83 e1 00 3c 	lwz     r31,60(r1)
	  34:	38 21 00 40 	addi    r1,r1,64
	  38:	48 00 00 00 	b       38 <ftrace_modify_code+0x38>
				38: R_PPC_REL24	patch_instruction

Before:

	   0:	94 21 ff c0 	stwu    r1,-64(r1)
	   4:	93 e1 00 3c 	stw     r31,60(r1)
	   8:	7c 7f 1b 79 	mr.     r31,r3
	   c:	40 80 00 4c 	bge     58 <ftrace_modify_code+0x58>
	  10:	93 c1 00 38 	stw     r30,56(r1)
	  14:	7c 9e 23 78 	mr      r30,r4
	  18:	7c a4 2b 78 	mr      r4,r5
	  1c:	80 bf 00 00 	lwz     r5,0(r31)
	  20:	7c 08 02 a6 	mflr    r0
	  24:	90 01 00 44 	stw     r0,68(r1)
	  28:	7c 1e 28 40 	cmplw   r30,r5
	  2c:	40 82 00 48 	bne     74 <ftrace_modify_code+0x74>
	  30:	7f e3 fb 78 	mr      r3,r31
	  34:	48 00 00 01 	bl      34 <ftrace_modify_code+0x34>
				34: R_PPC_REL24	patch_instruction
	  38:	80 01 00 44 	lwz     r0,68(r1)
	  3c:	20 63 00 00 	subfic  r3,r3,0
	  40:	83 c1 00 38 	lwz     r30,56(r1)
	  44:	7c 63 19 10 	subfe   r3,r3,r3
	  48:	7c 08 03 a6 	mtlr    r0
	  4c:	83 e1 00 3c 	lwz     r31,60(r1)
	  50:	38 21 00 40 	addi    r1,r1,64
	  54:	4e 80 00 20 	blr

It improves ftrace activation/deactivation duration by about 3%.

Modify patch_instruction() return on failure to -EPERM in order to
match with ftrace expectations. Other users of patch_instruction()
do not care about the exact error value returned.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/49a8597230713e2633e7d9d7b56140787c4a7e20.1652074503.git.christophe.leroy@csgroup.eu