Refactor zfs_context.h to build on FreeBSD

- on Linux move Linux specific headers to zfs_context_os.h
- on FreeBSD move FreeBSD specific definitions to zfs_context_os.h
- remove duplicate tsd_ definitions
- remove unused AT_TYPE

Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Don Brady <don.brady@delphix.com>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9668

include/os/linux/zfs/sys/Makefile.am

@@ -16,6 +16,7 @@ KERNEL_H = \
 	$(top_srcdir)/include/os/linux/zfs/sys/trace_zil.h \
 	$(top_srcdir)/include/os/linux/zfs/sys/trace_zio.h \
 	$(top_srcdir)/include/os/linux/zfs/sys/trace_zrlock.h \
+	$(top_srcdir)/include/os/linux/zfs/sys/zfs_context_os.h \
 	$(top_srcdir)/include/os/linux/zfs/sys/zfs_ctldir.h \
 	$(top_srcdir)/include/os/linux/zfs/sys/zfs_dir.h \
 	$(top_srcdir)/include/os/linux/zfs/sys/zfs_vfsops.h \

include/os/linux/zfs/sys/zfs_context_os.h

@@ -0,0 +1,30 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License, Version 1.0 only
+ * (the "License").  You may not use this file except in compliance
+ * with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://www.opensolaris.org/os/licensing.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+
+#ifndef ZFS_CONTEXT_OS_H
+#define	ZFS_CONTEXT_OS_H
+
+#include <sys/uio_impl.h>
+#include <linux/dcache_compat.h>
+#include <linux/utsname_compat.h>
+
+#endif