Revert changes to zbookmark_t

Commit 1421c89142 added a field to
zbookmark_t that unintentinoally caused a disk format change. This
negatively affected backward compatibility and platform portability.
Therefore, this field is being removed.

The function that field permitted is left unimplemented until a later
patch that will reimplement the field in a way that does not affect the
disk format.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2094

include/sys/zio.h

@@ -260,7 +260,6 @@ struct zbookmark {
 	uint64_t	zb_object;
 	int64_t		zb_level;
 	uint64_t	zb_blkid;
-	char *		zb_func;
 };
 
 #define	SET_BOOKMARK(zb, objset, object, level, blkid)  \
@@ -269,7 +268,6 @@ struct zbookmark {
 	(zb)->zb_object = object;                       \
 	(zb)->zb_level = level;                         \
 	(zb)->zb_blkid = blkid;                         \
-	(zb)->zb_func = FTAG;                           \
 }
 
 #define	ZB_DESTROYED_OBJSET	(-1ULL)