bcachefs: Add write buffer flush param to backpointer_get_key()

bcachefs: Add write buffer flush param to backpointer_get_key()

In an upcoming patch bch2_backpointer_get_key() will be repairing when
it finds a dangling backpointer; it will need to flush the btree write
buffer before it can definitively say there's an error.

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