bpf: dynamically allocate digest scratch buffer

Geert rightfully complained that 7bd509e3 ("bpf: add prog_digest
and expose it via fdinfo/netlink") added a too large allocation of
variable 'raw' from bss section, and should instead be done dynamically:

  # ./scripts/bloat-o-meter kernel/bpf/core.o.1 kernel/bpf/core.o.2
  add/remove: 3/0 grow/shrink: 0/0 up/down: 33291/0 (33291)
  function                                     old     new   delta
  raw                                            -   32832  +32832
  [...]

Since this is only relevant during program creation path, which can be
considered slow-path anyway, lets allocate that dynamically and be not
implicitly dependent on verifier mutex. Move bpf_prog_calc_digest() at
the beginning of replace_map_fd_with_map_ptr() and also error handling
stays straight forward.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>