bcachefs: Fix i_sectors_leak in bch2_truncate_page

When bch2_truncate_page() discards dirty sectors in the page cache, we
need to account for that - we don't need to account for allocated
sectors because that'll be done by the bch2_fpunch() call when it
updates the btree.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>