bch2_inode_delete_keys() was using BTREE_ITER_NOT_EXTENTS, on the
assumption that it would never need to split extents.

But that caused a race with extents being split by other threads -
specifically, the data move path. Extents iterators have the iterator
position pointing to the start of the extent, which avoids the race.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>