Make -d flag functional for gen_loader style program loading.

For example:
$ bpftool prog load -L -d test_d_path.o
... // will print:
libbpf: loading ./test_d_path.o
libbpf: elf: section(3) fentry/security_inode_getattr, size 280, link 0, flags 6, type=1
...
libbpf: prog 'prog_close': found data map 0 (test_d_p.bss, sec 7, off 0) for insn 30
libbpf: gen: load_btf: size 5376
libbpf: gen: map_create: test_d_p.bss idx 0 type 2 value_type_id 118
libbpf: map 'test_d_p.bss': created successfully, fd=0
libbpf: gen: map_update_elem: idx 0
libbpf: sec 'fentry/filp_close': found 1 CO-RE relocations
libbpf: record_relo_core: prog 1 insn[15] struct file 0:1 final insn_idx 15
libbpf: gen: prog_load: type 26 insns_cnt 35 progi_idx 0
libbpf: gen: find_attach_tgt security_inode_getattr 12
libbpf: gen: prog_load: type 26 insns_cnt 37 progi_idx 1
libbpf: gen: find_attach_tgt filp_close 12
libbpf: gen: finish 0
... // at this point libbpf finished generating loader program
   0: (bf) r6 = r1
   1: (bf) r1 = r10
   2: (07) r1 += -136
   3: (b7) r2 = 136
   4: (b7) r3 = 0
   5: (85) call bpf_probe_read_kernel#113
   6: (05) goto pc+104
... // this is the assembly dump of the loader program
 390: (63) *(u32 *)(r6 +44) = r0
 391: (18) r1 = map[idx:0]+5584
 393: (61) r0 = *(u32 *)(r1 +0)
 394: (63) *(u32 *)(r6 +24) = r0
 395: (b7) r0 = 0
 396: (95) exit
err 0  // the loader program was loaded and executed successfully
(null)
func#0 @0
...  // CO-RE in the kernel logs:
CO-RE relocating STRUCT file: found target candidate [500]
prog '': relo #0: kind <byte_off> (0), spec is [8] STRUCT file.f_path (0:1 @ offset 16)
prog '': relo #0: matching candidate #0 [500] STRUCT file.f_path (0:1 @ offset 16)
prog '': relo #0: patched insn #15 (ALU/ALU64) imm 16 -> 16
vmlinux_cand_cache:[11]file(500),
module_cand_cache:
... // verifier logs when it was checking test_d_path.o program:
R1 type=ctx expected=fp
0: R1=ctx(id=0,off=0,imm=0) R10=fp0
; int BPF_PROG(prog_close, struct file *file, void *id)
0: (79) r6 = *(u64 *)(r1 +0)
func 'filp_close' arg0 has btf_id 500 type STRUCT 'file'
1: R1=ctx(id=0,off=0,imm=0) R6_w=ptr_file(id=0,off=0,imm=0) R10=fp0
; pid_t pid = bpf_get_current_pid_tgid() >> 32;
1: (85) call bpf_get_current_pid_tgid#14

... // if there are multiple programs being loaded by the loader program
... // only the last program in the elf file will be printed, since
... // the same verifier log_buf is used for all PROG_LOAD commands.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211204194623.27779-1-alexei.starovoitov@gmail.com