Include sys/taskq.h in linux/vfs_compat.h

We should have included sys/taskq.h directly because we use the taskq
code here, but we instead had files that included sys/taskq.h also
include sys/kmem.h, which happened to include sys/taskq.h. sys/kmem.h no
longer does this, so we must define the include as we should
have done in the first place.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#2411

include/linux/vfs_compat.h

@@ -26,6 +26,8 @@
 #ifndef _ZFS_VFS_H
 #define	_ZFS_VFS_H
 
+#include <sys/taskq.h>
+
 /*
  * 2.6.28 API change,
  * Added insert_inode_locked() helper function, prior to this most callers