btrfs-progs: don't overrun "answer" array in cmds-chunk.c

btrfs-progs: don't overrun "answer" array in cmds-chunk.c

Eric noticed the trivial stack overflow bug in ask_user().  I went to
see the context for that fix and found that ask_user() was a bit much.

This fixes the overflow bug that Eric found, endless spinning on scanf()
errors, removes dead code, and leaves us with a trivial helper.

Signed-off-by: Zach Brown <zab@redhat.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>