btrfs: factor out the main loop of btrfs_buffered_write() into a helper

Inside the main loop of btrfs_buffered_write() we are doing a lot of
heavy lifting inside a while() loop.

This makes it pretty hard to read, factor out the content into a helper,
copy_one_range() to do the work.

This has no functional change, but with some minor variable renames,
e.g. rename all "sector" into "block".

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