bpf: Fix build_id caching in stack_map_get_build_id_offset()

This patch is a follow up to recent implementation of
stack_map_get_build_id_offset_sleepable() [1].

stack_map_get_build_id_offset() and its sleepable variant each cached
only the last successfully resolved VMA, with separate bookkeeping in
each function. A run of IPs in a VMA with no usable build ID will
repeat the lookup for every frame: find_vma() in the non-sleepable
path, a VMA lock and a blocking build_id_parse_file() in the sleepable.

Factor the per-call cache into a shared struct stack_map_build_id_cache
with two independent slots [2][3], used by both functions:

  * resolved   - last VMA that produced a build ID (file, build_id and
                 range), reused to skip the lookup and the parse;
  * unresolved - last VMA with no usable build ID (range only), reused to
                 emit a raw IP without another lookup or parse.

Keeping the slots independent means a build-ID-less VMA no longer evicts
the last resolved build ID, so a trace alternating between a binary and a
region without one stops re-resolving the binary on every return.

The shared lookup tests [vm_start, vm_end), matching the sleepable path;
the non-sleepable path previously reused a build ID for ip == vm_end
(range_in_vma() is inclusive) and now re-resolves it correctly.

[1] https://lore.kernel.org/bpf/20260525223948.1920986-1-ihor.solodrai@linux.dev/
[2] https://lore.kernel.org/bpf/CAEf4Bza2fRDGhLQoPE-EzM7F34xaEJfi5Exmxb-iWVUN3F06=g@mail.gmail.com/
[3] https://lore.kernel.org/bpf/CAEf4BzZXJFr=1iiVx937ht=4PYQkQHg=eFk810zhMDzXQG3ihw@mail.gmail.com/

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260615195536.1065107-1-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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