Add ioctl to support extended inode flags

Changes for v2:
- dropped renaming of variable from patch "btrfs: rename btrfs_flags_to_ioctl to reflect which flags it touches"
- fixed the error handling in "btrfs: add FS_IOC_FSSETXATTR ioctl"
- new patch to unify naming of some local variables

I'm about to add this patchset to the main queue for 4.18, unless there are objections.

The patchset implements the existing VFS ioctls for reading extended ioctl flags by btrfs.

There are many flags/attributes/extended/combined, the naming is confusing, so let's recap what we have:

* generic VFS inode flags (i_flags)
- S_* namespace
https://elixir.bootlin.com/linux/v4.17-rc1/source/include/linux/fs.h#L1850
- FS_IOC_GETFLAGS, FS_IOC_SETFLAGS
- tools: lsatrr, chattr

* btrfs inode flags, on-disk format, independent of the above, with to/from conversions
https://elixir.bootlin.com/linux/v4.17-rc1/source/fs/btrfs/ctree.h#L1416

* extended attributes, also called XATTR, but they're different entity, stored by an inode as key:value pairs
- tools: getfattr, setfattr

* XFLAGs, another interface to the generic S_* flags, new ioctl added because [GS]ETFLAGS is frozen, new bits added, eg. for project quotas or DAX, and more that originate in XFS features
https://elixir.bootlin.com/linux/latest/source/include/uapi/linux/fs.h#L168
- tools: xfs_io -c lsattr, xfs_io -c chattr

In the future, btrfs will probably get:

- nodefrag -- eg. to disable autodefrag or defrag ioctl
- nosymlinks -- for directories, prevent creating new symlinks
- dax

git://github.com/kdave/btrfs-devel dev/xflags

David Sterba (8):
btrfs: rename btrfs_update_iflags to reflect which flags it touches
btrfs: rename btrfs_mask_flags to reflect which flags it touches
btrfs: rename check_flags to reflect which flags it touches
btrfs: rename btrfs_flags_to_ioctl to reflect which flags it touches
btrfs: add helpers for FS_XFLAG_* conversion
btrfs: add FS_IOC_FSGETXATTR ioctl
btrfs: add FS_IOC_FSSETXATTR ioctl
btrfs: unify naming of flags variables for SETFLAGS and XFLAGS

 fs/btrfs/ctree.h |   2 +-
 fs/btrfs/inode.c |   4 +-
 fs/btrfs/ioctl.c | 271 ++++++++++++++++++++++++++++++++++++++++++-------------
 3 files changed, 213 insertions(+), 64 deletions(-)

2.16.2