Now that we have the logged operations btree, we can make
finsert/fcollapse atomic w.r.t. unclean shutdown as well.

This adds bch_logged_op_finsert to represent the state of an finsert or
fcollapse, which is a bit more complicated than truncate since we need
to track our position in the "shift extents" operation.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>