Convert function to use folios throughout. This is in preparation for the
removal of find_get_pages_range_tag(). This change removes 5 calls to
compound_head().
Initially the function was checking if the previous page index is truly the
previous page i.e. 1 index behind the current page. To convert to folios and
maintain this check we need to make the check
folio->index != prev + folio_nr_pages(previous folio) since we don't know
how many pages are in a folio.
At index i == 0 the check is guaranteed to succeed, so to workaround indexing
bounds we can simply ignore the check for that specific index. This makes the
initial assignment of prev trivial, so I removed that as well.
Also modified a comment in commit_checkpoint for consistency.
Signed-off-by: Vishal Moola (Oracle) <vishal.moola@xxxxxxxxx>