Share zfs_fsync, zfs_read, zfs_write, et al between Linux and FreeBSD

The zfs_fsync, zfs_read, and zfs_write function are almost identical
between Linux and FreeBSD.  With a little refactoring they can be
moved to the common code which is what is done by this commit.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #11078

lib/libspl/include/sys/uio.h

@@ -73,7 +73,6 @@ typedef struct uio {
 	uio_seg_t	uio_segflg;	/* address space (kernel or user) */
 	uint16_t	uio_fmode;	/* file mode flags */
 	uint16_t	uio_extflg;	/* extended flags */
-	offset_t	uio_limit;	/* u-limit (maximum byte offset) */
 	ssize_t		uio_resid;	/* residual count */
 } uio_t;