blktrace: make do_blk_trace_setup() static

This isn't used outside of blktrace.c anymore.

Fixes: 62c2a7d9

 ("block: push BKL into blktrace ioctls")
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>