bcachefs: bch2_btree_insert() - add btree iter flags

The commit 65bd4423

 ("bcachefs: bch2_btree_insert_trans() no longer
specifies BTREE_ITER_cached") removes BTREE_ITER_cached from
bch2_btree_insert_trans, which causes the update_inode function from
bcachefs-tools to take a long time (~20s).  Add an iter_flags parameter
to bch2_btree_insert, so the users can specify iter update trigger
flags, such as BTREE_ITER_cached.

Signed-off-by: Ariel Miculas <ariel.miculas@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>