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

module/zfs/zfs_ctldir.c

@@ -499,6 +499,9 @@ zfsctl_inode_alloc(zfs_sb_t *zsb, uint64_t id,
 	ip->i_ctime = now;
 	ip->i_fop = fops;
 	ip->i_op = ops;
+#if defined(IOP_XATTR)
+	ip->i_opflags &= ~IOP_XATTR;
+#endif
 
 	if (insert_inode_locked(ip)) {
 		unlock_new_inode(ip);