ARM: net: bpf: Improve prologue code sequence

Improve the prologue code sequence to be able to take advantage of
64-bit stores, changing the code from:

  push    {r4, r5, r6, r7, r8, r9, fp, lr}
  mov     fp, sp
  sub     ip, sp, #80     ; 0x50
  sub     sp, sp, #600    ; 0x258
  str     ip, [fp, #-100] ; 0xffffff9c
  mov     r6, #0
  str     r6, [fp, #-96]  ; 0xffffffa0
  mov     r4, #0
  mov     r3, r4
  mov     r2, r0
  str     r4, [fp, #-104] ; 0xffffff98
  str     r4, [fp, #-108] ; 0xffffff94

to the tighter:

  push    {r4, r5, r6, r7, r8, r9, fp, lr}
  mov     fp, sp
  mov     r3, #0
  sub     r2, sp, #80     ; 0x50
  sub     sp, sp, #600    ; 0x258
  strd    r2, [fp, #-100] ; 0xffffff9c
  mov     r2, #0
  strd    r2, [fp, #-108] ; 0xffffff94
  mov     r2, r0

resulting in a saving of three instructions.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/E1ieH2g-0004ih-Rb@rmk-PC.armlinux.org.uk