make W=1 generates the following warning in mmap_lock.c for allnoconfig

  mm/mmap_lock.c:213:6: warning: no previous prototype for `__mmap_lock_do_trace_start_locking' [-Wmissing-prototypes]
   void __mmap_lock_do_trace_start_locking(struct mm_struct *mm, bool write)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  mm/mmap_lock.c:219:6: warning: no previous prototype for `__mmap_lock_do_trace_acquire_returned' [-Wmissing-prototypes]
   void __mmap_lock_do_trace_acquire_returned(struct mm_struct *mm, bool write,
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  mm/mmap_lock.c:226:6: warning: no previous prototype for `__mmap_lock_do_trace_released' [-Wmissing-prototypes]
   void __mmap_lock_do_trace_released(struct mm_struct *mm, bool write)

On !CONFIG_TRACING configurations, the code is dead so put it behind an
#ifdef.

[cuibixuan@huawei.com: fix warning when CONFIG_TRACING is not defined]
  Link: https://lkml.kernel.org/r/20210531033426.74031-1-cuibixuan@huawei.com

Link: https://lkml.kernel.org/r/20210520084809.8576-13-mgorman@techsingularity.netSigned-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Bixuan Cui <cuibixuan@huawei.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>