bcachefs: Fix fsck warning about btree_trans not passed to fsck error

If a btree_trans is in use it's supposed to be passed to fsck_err so
that it can be unlocked if we're waiting on userspace input; but the
btree IO paths do call fsck errors where a btree_trans exists on the
stack but it's not passed through.

But it's ok, because it's unlocked while doing IO.

Fixes: a850bde6

 ("bcachefs: fsck_err() may now take a btree_trans")
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>