btrfs-progs: utils: flush stdout after prompt in ask_user()

When stdio is line buffered printf will not flush anything (on musl?),
leaving the program hanging without displaying any prompt and weird
dialogs such as the following:

  alpine:~# btrfstune -S 0 /dev/mmcblk1p1
  WARNING: this is dangerous, clearing the seeding flag may cause the derived device not to be mountable!
  y
  WARNING: seeding flag is not set on /dev/mmcblk1p1
  We are going to clear the seeding flag, are you sure? [y/N]: alpine:~#

Forcing flush makes the prompt display properly.

Signed-off-by: Dominique Martinet <dominique.martinet@atmark-techno.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>

common/utils.c

@@ -416,6 +416,7 @@ int ask_user(const char *question)
 	char *answer;
 
 	printf("%s [y/N]: ", question);
+	fflush(stdout);
 
 	return fgets(buf, sizeof(buf) - 1, stdin) &&
 	       (answer = strtok_r(buf, " \t\n\r", &saveptr)) &&