bcachefs: Fix UAFs of btree_insert_entry array

The btree paths array is now dynamically resizable - and as well the
btree_insert_entries array, as it needs to be the same size.

The merge path (and interior update path) allocates new btree paths,
thus can trigger a resize; thus we need to not retain direct pointers
after invoking merge; similarly when running btree node triggers.

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