btrfs: update stale comment in __cow_file_range_inline()

btrfs: update stale comment in __cow_file_range_inline()

We mention that the reserved data space is page size aligned but that's
not true anymore, as it's sector size aligned instead.
In commit 0bb067ca64e3 ("btrfs: fix the qgroup data free range for inline
data extents") we updated the amount passed to btrfs_qgroup_free_data()
from page size to sector size, but forgot to update the comment.

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