bpf: implement bpf to bpf calls support

bpf: implement bpf to bpf calls support

Implement missing bpf to bpf calls support. The loader will
recognize .text section and handle relocation entries that
are emitted by LLVM.

First step is processing of map related relocation entries
for .text section, and in a second step loader will copy .text
section into program section and adjust call instruction
offset accordingly.

Example with test_xdp_noinline.o from kernel selftests:

 1) Every function as __attribute__ ((always_inline)), rest
    left unchanged:

  # ip -force link set dev lo xdp obj test_xdp_noinline.o sec xdp-test
  # ip a
  1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 xdpgeneric/id:233 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
  [...]
  # bpftool prog dump xlated id 233
  [...]
  1669: (2d) if r3 > r2 goto pc+4
  1670: (79) r2 = *(u64 *)(r10 -136)
  1671: (61) r2 = *(u32 *)(r2 +0)
  1672: (63) *(u32 *)(r1 +0) = r2
  1673: (b7) r0 = 1
  1674: (95) exit        <-- 1674 insns total

 2) Every function as __attribute__ ((noinline)), rest
    left unchanged:

  # ip -force link set dev lo xdp obj test_xdp_noinline.o sec xdp-test
  # ip a
  1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 xdpgeneric/id:236 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
  [...]
  # bpftool prog dump xlated id 236
  [...]
  1000: (bf) r1 = r6
  1001: (b7) r2 = 24
  1002: (85) call pc+3   <-- pc-relative call insns
  1003: (1f) r7 -= r0
  1004: (bf) r0 = r7
  1005: (95) exit
  1006: (bf) r0 = r1
  1007: (bf) r1 = r2
  1008: (67) r1 <<= 32
  1009: (77) r1 >>= 32
  1010: (bf) r3 = r0
  1011: (6f) r3 <<= r1
  1012: (87) r2 = -r2
  1013: (57) r2 &= 31
  1014: (67) r0 <<= 32
  1015: (77) r0 >>= 32
  1016: (7f) r0 >>= r2
  1017: (4f) r0 |= r3
  1018: (95) exit        <-- 1018 insns total

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David Ahern <dsahern@gmail.com>