sync: latest libbpf changes from kernel

Syncing latest libbpf commits from kernel repository.
Baseline bpf-next commit:   a12978712d9001b060bcc10eaae42ad5102abe2b
Checkpoint bpf-next commit: ec5b8c76ab1c6d163762d60cfbedcd27e7527144
Baseline bpf commit:        b1c4b4d45263241ec6c2405a8df8265d4b58e707
Checkpoint bpf commit:      e1533b6319ab9c3a97dad314dd88b3783bc41b69

Alan Maguire (1):
  libbpf: Fix error handling in btf__distill_base()

Andreas Ziegler (1):
  libbpf: Add NULL checks to bpf_object__{prev_map,next_map}

Andrii Nakryiko (2):
  libbpf: fix BPF skeleton forward/backward compat handling
  libbpf: improve old BPF skeleton handling for map auto-attach

 src/btf.c    |  2 +-
 src/libbpf.c | 75 +++++++++++++++++++++++++++++-----------------------
 2 files changed, 43 insertions(+), 34 deletions(-)

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

BPF-CHECKPOINT-COMMIT

@@ -1 +1 @@
-b1c4b4d45263241ec6c2405a8df8265d4b58e707
+e1533b6319ab9c3a97dad314dd88b3783bc41b69

CHECKPOINT-COMMIT

@@ -1 +1 @@
-a12978712d9001b060bcc10eaae42ad5102abe2b
+ec5b8c76ab1c6d163762d60cfbedcd27e7527144