Kernel has removed support for this feature in 5.7 so let's remove
support from progs as well.

Signed-off-by: Nikolay Borisov <redacted>
---

This has been languishing on my local branch, let's merge it and be done with
it once for all.

 ioctl.h                                     |  4 +--
 libbtrfsutil/README.md                      | 14 ++------
 libbtrfsutil/btrfs.h                        |  4 +--
 libbtrfsutil/btrfsutil.h                    | 23 +++++--------
 libbtrfsutil/python/module.c                |  6 ++--
 libbtrfsutil/python/tests/test_subvolume.py | 12 ++-----
 libbtrfsutil/subvolume.c                    | 38 ++++++---------------
 7 files changed, 29 insertions(+), 72 deletions(-)

diff --git a/ioctl.h b/ioctl.h
index 5ce47c838b0e..aad3e9772908 100644
--- a/ioctl.h
+++ b/ioctl.h

@@ -49,15 +49,13 @@ BUILD_ASSERT(sizeof(struct btrfs_ioctl_vol_args) == 4096);

 #define BTRFS_DEVICE_PATH_NAME_MAX 1024

-#define BTRFS_SUBVOL_CREATE_ASYNC	(1ULL << 0)