ARM: net: bpf: provide accessor functions for BPF registers

Many of the code paths need to have knowledge about whether a register
is stacked or in a CPU register.  Move this decision making to a pair
of helper functions instead of having it scattered throughout the
code.
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>