Linux 4.9 compat: fix zfs_ctldir xattr handling

Since torvalds/linux@d0a5b99 IOP_XATTR is used to indicate the inode
has xattr support: clear it for the ctldir inodes to avoid EIO errors.

Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6189