btrfs: rename variables for locked range in defrag_prepare_one_folio()

btrfs: rename variables for locked range in defrag_prepare_one_folio()

In preparation to use a helper for folio_pos + folio_size, rename the
variables for the locked range so they don't use the 'folio_' prefix. As
the locking ranges take inclusive end of the range (hence the "-1") this
would be confusing as the folio helpers typically use exclusive end of
the range.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>