With snapshots, __bch2_dev_usr_data_drop() now uses an ALL_SNAPSHOTS
iterator, which isn't an extent iterator - meaning we shouldn't be
inserting whiteouts with nonzero size to delete. This fixes a bug where
we go RO because we tried to insert an invalid key in the device remove
path.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>