Introduce ring__consume_n() and ring_buffer__consume_n() API to
partially consume items from one (or more) ringbuffer(s).

This can be useful, for example, to consume just a single item or when
we need to copy multiple items to a limited user-space buffer from the
ringbuffer callback.

Practical example (where this API can be used):
https://github.com/sched-ext/scx/blob/b7c06b9ed9f72cad83c31e39e9c4e2cfd8683a55/rust/scx_rustland_core/src/bpf.rs#L217

See also:
https://lore.kernel.org/lkml/20240310154726.734289-1-andrea.righi@canonical.com/T/#u

v4:
 - add a selftest to test the new API
 - open a new 1.5.0 cycle

v3:
 - rename ring__consume_max() -> ring__consume_n() and
   ring_buffer__consume_max() -> ring_buffer__consume_n()
 - add new API to a new 1.5.0 cycle
 - fixed minor nits / comments

v2:
 - introduce a new API instead of changing the callback's retcode
   behavior

Andrea Righi (4):
      libbpf: Start v1.5 development cycle
      libbpf: ringbuf: allow to consume up to a certain amount of items
      libbpf: Add ring__consume_n / ring_buffer__consume_n
      selftests/bpf: Add tests for ring__consume_n and ring_buffer__consume_n

 tools/lib/bpf/libbpf.h                           | 12 +++++
 tools/lib/bpf/libbpf.map                         |  6 +++
 tools/lib/bpf/libbpf_version.h                   |  2 +-
 tools/lib/bpf/ringbuf.c                          | 59 ++++++++++++++++++++----
 tools/testing/selftests/bpf/prog_tests/ringbuf.c |  8 ++++
 5 files changed, 76 insertions(+), 11 deletions(-)