fs/xfs: Support that ioctl(SETXFLAGS/GETXFLAGS) can set/get inode DAX on XFS.

1) FS_DAX_FL has been introduced by commit b383a73f

.
2) In future, chattr/lsattr command from e2fsprogs can set/get
   inode DAX on XFS by calling ioctl(SETXFLAGS/GETXFLAGS).

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>