bpf: bpf_prog_pack: Set proper size before freeing ro_header

bpf_prog_pack_free() uses header->size to decide whether the header
should be freed with module_memfree() or the bpf_prog_pack logic.
However, in kvmalloc() failure path of bpf_jit_binary_pack_alloc(),
header->size is not set yet. As a result, bpf_prog_pack_free() may treat
a slice of a pack as a standalone kvmalloc'd header and call
module_memfree() on the whole pack. This in turn causes use-after-free by
other users of the pack.

Fix this by setting ro_header->size before freeing ro_header.

Fixes: 33c98058 ("bpf: Introduce bpf_jit_binary_pack_[alloc|finalize|free]")
Reported-by:  <syzbot+2f649ec6d2eea1495a8f@syzkaller.appspotmail.com>
Reported-by:  <syzbot+ecb1e7e51c52f68f7481@syzkaller.appspotmail.com>
Reported-by:  <syzbot+87f65c75f4a72db05445@syzkaller.appspotmail.com>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220217183001.1876034-1-song@kernel.org