powerpc/64: Force inlining of prevent_user_access() and set_kuap()

A ppc64_defconfig build exhibits about 10 copied of
prevent_user_access(). It also have one copy of set_kuap().

	c000000000017340 <.prevent_user_access.constprop.0>:
	c00000000001a038:	4b ff d3 09 	bl      c000000000017340 <.prevent_user_access.constprop.0>
	c00000000001aabc:	4b ff c8 85 	bl      c000000000017340 <.prevent_user_access.constprop.0>
	c00000000001ab38:	4b ff c8 09 	bl      c000000000017340 <.prevent_user_access.constprop.0>
	c00000000001ade0:	4b ff c5 61 	bl      c000000000017340 <.prevent_user_access.constprop.0>
	c000000000039b90 <.prevent_user_access.constprop.0>:
	c00000000003ac08:	4b ff ef 89 	bl      c000000000039b90 <.prevent_user_access.constprop.0>
	c00000000003b9d0:	4b ff e1 c1 	bl      c000000000039b90 <.prevent_user_access.constprop.0>
	c00000000003ba54:	4b ff e1 3d 	bl      c000000000039b90 <.prevent_user_access.constprop.0>
	c00000000003bbfc:	4b ff df 95 	bl      c000000000039b90 <.prevent_user_access.constprop.0>
	c00000000015dde0 <.prevent_user_access.constprop.0>:
	c0000000001612c0:	4b ff cb 21 	bl      c00000000015dde0 <.prevent_user_access.constprop.0>
	c000000000161b54:	4b ff c2 8d 	bl      c00000000015dde0 <.prevent_user_access.constprop.0>
	c000000000188cf0 <.prevent_user_access.constprop.0>:
	c00000000018d658:	4b ff b6 99 	bl      c000000000188cf0 <.prevent_user_access.constprop.0>
	c00000000030fe20 <.prevent_user_access.constprop.0>:
	c0000000003123d4:	4b ff da 4d 	bl      c00000000030fe20 <.prevent_user_access.constprop.0>
	c000000000313970:	4b ff c4 b1 	bl      c00000000030fe20 <.prevent_user_access.constprop.0>
	c0000000005e6bd0 <.prevent_user_access.constprop.0>:
	c0000000005e7d8c:	4b ff ee 45 	bl      c0000000005e6bd0 <.prevent_user_access.constprop.0>
	c0000000007bcae0 <.prevent_user_access.constprop.0>:
	c0000000007bda10:	4b ff f0 d1 	bl      c0000000007bcae0 <.prevent_user_access.constprop.0>
	c0000000007bda54:	4b ff f0 8d 	bl      c0000000007bcae0 <.prevent_user_access.constprop.0>
	c0000000007bdd28:	4b ff ed b9 	bl      c0000000007bcae0 <.prevent_user_access.constprop.0>
	c0000000007c0390:	4b ff c7 51 	bl      c0000000007bcae0 <.prevent_user_access.constprop.0>
	c00000000094e4f0 <.prevent_user_access.constprop.0>:
	c000000000950e40:	4b ff d6 b1 	bl      c00000000094e4f0 <.prevent_user_access.constprop.0>
	c00000000097d2d0 <.prevent_user_access.constprop.0>:
	c0000000009813fc:	4b ff be d5 	bl      c00000000097d2d0 <.prevent_user_access.constprop.0>
	c000000000acd540 <.prevent_user_access.constprop.0>:
	c000000000ad1d60:	4b ff b7 e1 	bl      c000000000acd540 <.prevent_user_access.constprop.0>
	c000000000e5d680 <.prevent_user_access.constprop.0>:
	c000000000e64b60:	4b ff 8b 21 	bl      c000000000e5d680 <.prevent_user_access.constprop.0>
	c000000000e64b6c:	4b ff 8b 15 	bl      c000000000e5d680 <.prevent_user_access.constprop.0>
	c000000000e64c38:	4b ff 8a 49 	bl      c000000000e5d680 <.prevent_user_access.constprop.0>

When building signal_64.c with -Winline the following messages appear:

	./arch/powerpc/include/asm/book3s/64/kup.h:331:20: error: inlining failed in call to 'set_kuap': call is unlikely and code size would grow [-Werror=inline]
	./arch/powerpc/include/asm/book3s/64/kup.h:401:20: error: inlining failed in call to 'prevent_user_access.constprop': call is unlikely and code size would grow [-Werror=inline]

Those functions are used on hot pathes and have been
expected to be inlined at all time.

Force them inline.

This patch reduces the kernel text size by 700 bytes, confirming
that not inlining those functions is not worth it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/eff9b2b211957fa2e8707e46f31674097fd563a3.1644588972.git.christophe.leroy@csgroup.eu