proc: smaller readlock section in readdir("/proc")

Readdir context is thread local, so ->pos is thread local,
move it out of readlock.

Link: http://lkml.kernel.org/r/20180627195339.GD18113@avx2
Signed-off-by: Alexey Dobriyan <[e-mail]>
Reviewed-by: Andrew Morton <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/proc/generic.c

···
 	if (!dir_emit_dots(file, ctx))
 		return 0;
 
+	i = ctx->pos - 2;
 	read_lock(&proc_subdir_lock);
 	de = pde_subdir_first(de);
-	i = ctx->pos - 2;
 	for (;;) {
 		if (!de) {
 			read_unlock(&proc_subdir_lock);
···
 			pde_put(de);
 			return 0;
 		}
-		read_lock(&proc_subdir_lock);
 		ctx->pos++;
+		read_lock(&proc_subdir_lock);
 		next = pde_subdir_next(de);
 		pde_put(de);
 		de = next;