btrfs: remove force_page_uptodate variable from btrfs_buffered_write()

Commit c87c2997 ("btrfs: make buffered write to copy one page a
time") changed how the variable @force_page_uptodate was updated.

Before that commit the variable was only initialized to false at the
beginning of the function, and after hitting a short copy, the next
retry on the same folio would force the folio to be read from the disk.

But after the commit, the variable is always initialized to false at the
beginning of the loop's scope, causing prepare_one_folio() never to get a
true value passed in.

The change in behavior is not a huge deal, it only makes a difference
on how we handle short copies:

Old: Allow the buffer to be split

     The first short copy will be rejected, that's the same for both
     cases.

     But for the next retry, we require the folio to be read from disk.

     Then even if we hit a short copy again, since the folio is already
     uptodate, we do not need to handle partial uptodate range, and can
     continue, marking the short copied range as dirty and continue.

     This will split the buffer write into the folio as two buffered
     writes.

New: Do not allow the buffer to be split

     The first short copy will be rejected, that's the same for both
     cases.

     For the next retry, we do nothing special, thus if the short copy
     happened again, we reject it again, until either the short copy is
     gone, or we failed to fault in the buffer.

     This will mean the buffer write into the folio will either fail or
     succeed, no splitting will happen.

To me, either solution is fine, but the new one makes it simpler and
requires no special handling, so I prefer that solution.

And since @force_page_uptodate is always false when passed into
prepare_one_folio(), we can just remove the variable.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>