Key a splat by its source line and not the function it was inlined into

[tamnd]

On 6.12.111 X64, rk test failed the rucc kernel on one splat that only its log had:

WARNING: CPU: N PID: N at drivers/gpu/drm/drm_rect.c:137 drm_calc_scale+N

The reference log has the same warning at the same line, the same number of times, from the same KUnit cases (negative src and negative dst in drm_rect_test expect the WARN_ON). The only difference is the function name. GCC inlines the static drm_calc_scale into drm_rect_calc_hscale and drm_rect_calc_vscale, and rucc keeps it as a function, so the reference key ends in drm_rect_calc_hscale+N.

The function after a source location tells which function the code was inlined into, and that is a compiler choice. This change drops it from the normalized first line when a source location comes before it. The source location stays, so a warning at a different line is still a different splat. Lines with no source location, such as BUG: unable to handle page fault, do not change.

Tested on server2: cargo fmt --check -p rk, cargo clippy -p rk --all-targets -- -D warnings and cargo test -p rk (136 passed, with a new test for this case).