Merge branch 'libbpf: add legacy uprobe support'

Andrii Nakryiko says:

====================

Implement libbpf support for attaching uprobes/uretprobes using legacy
tracefs interfaces. This is a logical complement to recently landed legacy
kprobe support ([0]). This patch refactors existing legacy kprobe code to be more
uniform with uprobe code as well, making the logic easier to compare and
follow.

This patch set also fixes two bugs recently found by Coverity in legacy kprobe
handling code, and thus subsumes previously submitted two patches ([1]):
original patch #1 is kept as is, while original patch #2 was dropped because
patch #3 of the current series refactors and fixes affected code.

  [0] https://patchwork.kernel.org/project/netdevbpf/patch/20210912064844.3181742-1-rafaeldtinoco@gmail.com/
  [1] https://patchwork.kernel.org/project/netdevbpf/list/?series=549977&state=*

v1->v2:
  - drop 'legacy = true' debug left-over and explain legacy check (Alexei).
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>