Function write_extent_to_disk() is just writing the content of a tree
block to disk.
It can not handle RAID56, and its work is the same as
write_data_to_disk().
Thus we can replace write_extent_to_disk() with write_data_to_disk()
easily.
There is only one special call site in write_raid56_with_parity(), which
can easily be replace with btrfs_pwrite() directly.
This reduce the write entrance, and make later eb::fd removal easier.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
