scheduler: fix SMP waitpid reap race and harden socket file init

scheduler: fix SMP waitpid reap race and harden socket file init

Prevent process_waitpid() from reaping a zombie child while it is still the current process on another CPU. This avoids freeing a live process struct and kernel stack during the exit-to-schedule handoff under SMP, which matched the intermittent invalid-opcode panic seen after the job-control path in fulltest.

Also zero-initialize struct file allocations in the socket() and accept() paths so mount_root and any future fields cannot inherit heap garbage.

Validation: make -j$(nproc), make test-host, make test SMOKE_SMP=4, make test-battery (153/153 PASS).