libbpf: make RINGBUF map size adjustments more eagerly

libbpf: make RINGBUF map size adjustments more eagerly

Make libbpf adjust RINGBUF map size (rounding it up to closest power-of-2
of page_size) more eagerly: during open phase when initializing the map
and on explicit calls to bpf_map__set_max_entries().

Such approach allows user to check actual size of BPF ringbuf even
before it's created in the kernel, but also it prevents various edge
case scenarios where BPF ringbuf size can get out of sync with what it
would be in kernel. One of them (reported in [0]) is during an attempt
to pin/reuse BPF ringbuf.

Move adjust_ringbuf_sz() helper closer to its first actual use. The
implementation of the helper is unchanged.

Also make detection of whether bpf_object is already loaded more robust
by checking obj->loaded explicitly, given that map->fd can be < 0 even
if bpf_object is already loaded due to ability to disable map creation
with bpf_map__set_autocreate(map, false).

  [0] Closes: https://github.com/libbpf/libbpf/pull/530

Fixes: 0087a681fa8c ("libbpf: Automatically fix up BPF_MAP_TYPE_RINGBUF size, if necessary")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220715230952.2219271-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>