__do_irq() inconditionnaly calls ppc_md.get_irq()

That's definitely a hot path.

At the time being ppc_md.get_irq address is read every time
from ppc_md structure.

Replace that call by a static call, and initialise that
call after ppc_md.init_IRQ() has set ppc_md.get_irq.

Emit a warning and don't set the static call if ppc_md.init_IRQ()
is still NULL, that way the kernel won't blow up if for some
reason ppc_md.get_irq() doesn't get properly set.

With the patch:

	00000000 <__SCT__ppc_get_irq>:
	   0:	48 00 00 20 	b       20 <__static_call_return0>	<== Replaced by 'b <ppc_md.get_irq>' at runtime
...
	00000020 <__static_call_return0>:
	  20:	38 60 00 00 	li      r3,0
	  24:	4e 80 00 20 	blr
...
	00000058 <__do_irq>:
...
	  64:	48 00 00 01 	bl      64 <__do_irq+0xc>
				64: R_PPC_REL24	__SCT__ppc_get_irq
	  68:	2c 03 00 00 	cmpwi   r3,0
...

Before the patch:

	00000038 <__do_irq>:
...
	  3c:	3d 20 00 00 	lis     r9,0
				3e: R_PPC_ADDR16_HA	ppc_md+0x1c
...
	  44:	81 29 00 00 	lwz     r9,0(r9)
				46: R_PPC_ADDR16_LO	ppc_md+0x1c
...
	  4c:	7d 29 03 a6 	mtctr   r9
	  50:	4e 80 04 21 	bctrl
	  54:	2c 03 00 00 	cmpwi   r3,0
...

On PPC64 which doesn't implement static calls yet we get:

00000000000000d0 <__do_irq>:
...
      dc:	00 00 22 3d 	addis   r9,r2,0
			dc: R_PPC64_TOC16_HA	.data+0x8
...
      e4:	00 00 89 e9 	ld      r12,0(r9)
			e4: R_PPC64_TOC16_LO_DS	.data+0x8
...
      f0:	a6 03 89 7d 	mtctr   r12
      f4:	18 00 41 f8 	std     r2,24(r1)
      f8:	21 04 80 4e 	bctrl
      fc:	18 00 41 e8 	ld      r2,24(r1)
...

So on PPC64 that's similar to what we get without static calls.
But at least until ppc_md.get_irq() is set the call is to
__static_call_return0.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/afb92085f930651d8b1063e4d4bf0396c80ebc7d.1647002274.git.christophe.leroy@csgroup.eu