btrfs: Remove EXTENT_IOBITS

This flag just became synonymous to EXTENT_LOCKED, so just remove it and
used EXTENT_LOCKED directly. No functional changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>