Fix loop device becomes read-only

Commit 933ec99 removes read and write from f_op because the vfs layer will
select iter_write or aio_write automatically. However, for Linux <= 4.0,
loop_set_fd will actually check f_op->write and set read-only if not exists.
This patch add them back and use the generic do_sync_{read,write} for
aio_{read,write} and new_sync_{read,write} for {read,write}_iter.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes #5776 
Closes #5855

module/zfs/zpl_file.c

@@ -856,9 +856,15 @@ const struct file_operations zpl_file_operations = {
 	.release	= zpl_release,
 	.llseek		= zpl_llseek,
 #ifdef HAVE_VFS_RW_ITERATE
+#ifdef HAVE_NEW_SYNC_READ
+	.read		= new_sync_read,
+	.write		= new_sync_write,
+#endif
 	.read_iter	= zpl_iter_read,
 	.write_iter	= zpl_iter_write,
 #else
+	.read		= do_sync_read,
+	.write		= do_sync_write,
 	.aio_read	= zpl_aio_read,
 	.aio_write	= zpl_aio_write,
 #endif