Restructure uio to accommodate bio_vec

Starting from Linux 4.1, bio_vec will be allowed to pass into filesystem via
iter_read/iter_write, so we add a bio_vec field in uio_t to hold it, and use
UIO_BVEC in segflg to determine which "vec".

Also, to be consistent to newer kernel, we make iovec and bio_vec immutable,
and make uio act as an iterator with the new uio_skip field indicating number
of bytes to skip in the first segment.

Signed-off-by: Chunwei Chen <tuxoko@gmail.com>

include/sys/uio.h

@@ -1,6 +1,7 @@
 /*****************************************************************************\
  *  Copyright (C) 2007-2010 Lawrence Livermore National Security, LLC.
  *  Copyright (C) 2007 The Regents of the University of California.
+ *  Copyright (c) 2015 by Chunwei Chen. All rights reserved.
  *  Produced at Lawrence Livermore National Laboratory (cf, DISCLAIMER).
  *  Written by Brian Behlendorf <behlendorf1@llnl.gov>.
  *  UCRL-CODE-235197
@@ -40,17 +41,22 @@ typedef enum uio_seg {
 	UIO_USERSPACE =	0,
 	UIO_SYSSPACE =	1,
 	UIO_USERISPACE=	2,
+	UIO_BVEC =	3,
 } uio_seg_t;
 
 typedef struct uio {
-	struct iovec	*uio_iov;
+	union {
+		const struct iovec	*uio_iov;
+		const struct bio_vec	*uio_bvec;
+	};
 	int		uio_iovcnt;
 	offset_t	uio_loffset;
 	uio_seg_t	uio_segflg;
 	uint16_t	uio_fmode;
 	uint16_t	uio_extflg;
 	offset_t	uio_limit;
 	ssize_t		uio_resid;
+	size_t		uio_skip;
 } uio_t;
 
 typedef struct aio_req {