bcachefs: Fix failure to allocate btree node in cache

bcachefs: Fix failure to allocate btree node in cache

The error code when we fail to allocate a node in the btree node cache
doesn't make it to bch2_btree_path_traverse_all(). Instead, we need to
stash a flag in btree_trans so we know we have to take the cannibalize
lock.

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