linux/efa: Fix config probe race with file-based completion

[mjonuschat]

Issue #, if available: #364

Description of changes:

Fix config probe race condition by replacing PID polling with file-based completion signaling

The EFA driver's cmake-based config probing launches compile_conftest.sh processes in the background
via nohup in runbg.sh and tracks them by polling /proc/$pid. The nohup wrapper exits before
the actual compile and config.h write completes, causing the PID to disappear from /proc
prematurely. On machines with high core counts this results in config.h missing a significant number
of defines, leading to build failures or a misconfigured driver module.

Replace PID polling with file-based completion signaling. Each background probe now touches a
done-file on completion, and the waiter polls for that file instead. Config probes continue to run in
parallel. Renamed wait_for_pid to wait_for_completion and pids to pending throughout to
reflect the new mechanism.

Testing

Built against kernel 6.16 on a 72-CPU machine. A known-good baseline (57 defines) was established using a synchronous build. The fix was then verified over 10 consecutive builds, all producing identical sorted config.h output (10/10
PASS). PID polling consistently produced only 30-33 out of 57 defines under the same conditions.

Approach	Defines	Expected	Result
PID polling (before)	30-33	57	FAIL
File-based completion signaling (this fix)	57	57	PASS (10/10)

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