profiling: fix broken profiling regression

Impact: fix broken /proc/profile on UP machines

Commit c309b917cab55799ea489d7b5f1b77025d9f8462 "cpumask: convert
kernel/profile.c" broke profiling.  prof_cpu_mask was previously
initialized to CPU_MASK_ALL, but left uninitialized in that commit.
We need to copy cpu_possible_mask (cpu_online_mask is not enough).

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>