In xfs_buffered_write_iomap_begin, @icur is the iext cursor for the data
fork and @ccur is the cursor for the cow fork. Pass in whichever cursor
corresponds to allocfork, because otherwise the xfs_iext_prev_extent
call can use the data fork cursor to walk off the end of the cow fork
structure. Best case it returns the wrong results, worst case it does
this: