bcachefs: Change btree wb assert to runtime error

We just had a report of the assert for "btree in write buffer for
non-write buffer btree" popping during the 6.14 upgrade.

- 150TB filesystem, after a reboot the upgrade was able to continue from
  where it left off, so no major damage.

But with 6.14 about to come out we want to get this tracked down asap,
and need more data if other users hit this.

Convert the BUG_ON() to an emergency read-only, and print out btree, the
key itself, and stack trace from the original write buffer update (which
did not have this check before).

Reported-by: Stijn Tintel <stijn@linux-ipv6.be>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>