nfp: bpf: fix return address from register-saving subroutine to callee

On performing a BPF-to-BPF call, we first jump to a subroutine that
pushes callee-saved registers (R6~R9) to the stack, and from there we
goes to the start of the callee next. In order to do so, the caller must
pass to the subroutine the address of the NFP instruction to jump to at
the end of that subroutine. This cannot be reliably implemented when
translated the caller, as we do not always know the start offset of the
callee yet.

This patch implement the required fixup step for passing the start
offset in the callee via the register used by the subroutine to hold its
return address.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>