ARM: net: bpf: access eBPF scratch space using ARM FP register

Access the eBPF scratch space using the frame pointer rather than our
stack pointer, as the offsets from the ARM frame pointer are constant
across all eBPF programs.

Since we no longer reference the scratch space registers from the stack
pointer, this simplifies emit_push_r64() as it no longer needs to know
how many words are pushed onto the stack.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>