btree nodes can be written by other threads (shrinker, journal reclaim)
with only a read lock, but brand new nodes should only be written by the
thread doing the split/interior update. bch2_btree_update_add_new_node()
sets btree node flags to indicate that this is a new node and should not
be written out by other threads, thus we need to call it before dropping
our write lock.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>