Previously, we were doing btree node merging from
bch2_btree_insert_node() - but this is called from the split path, when
we're in the middle of creating new nodes and deleting new nodes and the
iterators are in a weird state.

Also, this means we're starting a new btree_update while in the middle
of an existing one, and that's asking for deadlocks.

Much simpler and saner to trigger btree node merging _after_ the whole
btree node split path is finished.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>