Inode creation is done with non-cached btree iterators, but then in the
same transaction the inode may be updated again with a cached iterator -
it makes cache coherency easier if new inodes always land in the
underlying btree.

This patch adds a check to bch2_trans_update() - if the same key is
updated multiple times in the same transaction with both cached and non
cache iterators, use the non cached iterator.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>