bcachefs: Don't trust sb->nr_devices in members_to_text()

We have to be able to print superblock sections even if they fail to
validate (for debugging), so we have to calculate the number of entries
from the field size.

Reported-by:  <syzbot+5138f00559ffb3cb3610@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>