objpool: enable inlining objpool_push() and objpool_pop() operations

objpool_push() and objpool_pop() are very performance-critical functions
and can be called very frequently in kretprobe triggering path.

As such, it makes sense to allow compiler to inline them completely to
eliminate function calls overhead. Luckily, their logic is quite well
isolated and doesn't have any sprawling dependencies.

This patch moves both objpool_push() and objpool_pop() into
include/linux/objpool.h and marks them as static inline functions,
enabling inlining. To avoid anyone using internal helpers
(objpool_try_get_slot, objpool_try_add_slot), rename them to use leading
underscores.

We used kretprobe microbenchmark from BPF selftests (bench trig-kprobe
and trig-kprobe-multi benchmarks) running no-op BPF kretprobe/kretprobe.multi
programs in a tight loop to evaluate the effect. BPF own overhead in
this case is minimal and it mostly stresses the rest of in-kernel
kretprobe infrastructure overhead. Results are in millions of calls per
second. This is not super scientific, but shows the trend nevertheless.

BEFORE
======
kretprobe      :    9.794 ± 0.086M/s
kretprobe-multi:   10.219 ± 0.032M/s

AFTER
=====
kretprobe      :    9.937 ± 0.174M/s (+1.5%)
kretprobe-multi:   10.440 ± 0.108M/s (+2.2%)

Link: https://lore.kernel.org/all/20240424215214.3956041-2-andrii@kernel.org/

Cc: Matt (Qiang) Wu <wuqiang.matt@bytedance.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>