Btrfs: remove no longer used io_err from btrfs_log_ctx

Btrfs: remove no longer used io_err from btrfs_log_ctx

The io_err field of struct btrfs_log_ctx is no longer used after the
recent simplification of the fast fsync path, where we now wait for
ordered extents to complete before logging the inode. We did this in
commit 5ab04a544cca ("btrfs: always wait on ordered extents at fsync
time") and commit 5b3f708e964b ("btrfs: clean up the left over
logged_list usage") removed its last use.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>