Prefix struct rangelock

A struct rangelock already exists on FreeBSD.  Add a zfs_ prefix as
per our convention to prevent any conflict with existing symbols.
This change is a follow up to 2cc479d0.

Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9534

include/sys/zfs_znode.h

@@ -191,7 +191,7 @@ typedef struct znode {
 	krwlock_t	z_parent_lock;	/* parent lock for directories */
 	krwlock_t	z_name_lock;	/* "master" lock for dirent locks */
 	zfs_dirlock_t	*z_dirlocks;	/* directory entry lock list */
-	rangelock_t	z_rangelock;	/* file range locks */
+	zfs_rangelock_t	z_rangelock;	/* file range locks */
 	boolean_t	z_unlinked;	/* file has been unlinked */
 	boolean_t	z_atime_dirty;	/* atime needs to be synced */
 	boolean_t	z_zn_prefetch;	/* Prefetch znodes? */