bcachefs: Fix NULL pointer dereference in bch2_opt_to_text

This patch adds a bounds check to the bch2_opt_to_text function to prevent
NULL pointer dereferences when accessing the opt->choices array. This
ensures that the index used is within valid bounds before dereferencing.
The new version enhances the readability.

Reported-and-tested-by:  <syzbot+37186860aa7812b331d5@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=37186860aa7812b331d5


Signed-off-by: Mohammed Anees <pvmohammedanees2003@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>