fs: XFS_IOC_FS[SG]SETXATTR to FS_IOC_FS[SG]ETXATTR promotion

Hoist the ioctl definitions for the XFS_IOC_FS[SG]SETXATTR API from
fs/xfs/libxfs/xfs_fs.h to include/uapi/linux/fs.h so that the ioctls
can be used by all filesystems, not just XFS. This enables
(initially) ext4 to use the ioctl to set project IDs on inodes.

Based-on-patch-from: Li Xi <lixi@ddn.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>