bcachefs: Fix race between journal_seq_copy() and journal_seq_drop()

bcachefs: Fix race between journal_seq_copy() and journal_seq_drop()

In bch2_btree_interior_update_will_free_node, we copy the journal pins
from outstanding writes on the btree node we're about to free. But, this
can race with the writes completing, and dropping their journal pins.

To guard against this, just use READ_ONCE() in bch2_journal_pin_copy().

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