bcachefs: bch2_extent_update_i_size_sectors()

bcachefs: bch2_extent_update_i_size_sectors()

In the io path, when we do the extent update we also have to update the
inode - for i_size and i_sectors updates, as well as for bi_journal_seq
for fsync.

This factors that out into a new helper which will be used in the new
nocow mode, in the unwritten extent conversion path.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>