ARM: net: bpf: fix register saving

When an eBPF program tail-calls another eBPF program, it enters it after
the prologue to avoid having complex stack manipulations.  This can lead
to kernel oopses, and similar.

Resolve this by always using a fixed stack layout, a CPU register frame
pointer, and using this when reloading registers before returning.

Fixes: 39c13c20 ("arm: eBPF JIT compiler")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>