bpf: Remove unnecessary assertion on fp_old

The two callers of bpf_prog_realloc - bpf_patch_insn_single and
bpf_migrate_filter dereference the struct fp_old, before passing
it to the function. Thus assertion to check fp_old is unnecessary
and can be removed.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191219175735.19231-1-pakki001@umn.edu