selftests/bpf: Add tests for kprobe.session optimization

selftests/bpf: Add tests for kprobe.session optimization

Extend existing kprobe_multi_test subtests to validate the
kprobe.session exact function name optimization:

In kprobe_multi_session.c, add test_kprobe_syms which attaches a
kprobe.session program to an exact function name (bpf_fentry_test1)
exercising the fast syms[] path that bypasses kallsyms parsing.  It
calls session_check() so bpf_fentry_test1 is hit by both the wildcard
and exact probes, and test_session_skel_api validates
kprobe_session_result[0] == 4 (entry + return from each probe).

In test_attach_api_fails, add fail_7 and fail_8 verifying error code
consistency between the wildcard pattern path (slow, parses kallsyms)
and the exact function name path (fast, uses syms[] array).  Both
paths must return -ENOENT for non-existent functions.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@crowdstrike.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260302200837.317907-4-andrey.grodzovsky@crowdstrike.com