The abd child/parent relationship does not need to be tracked

ABD's currently track their parent/child relationship.  This applies to
`abd_get_offset()` and `abd_borrow_buf()`.  However, nothing depends on
knowing this relationship, it's only used for consistency checks to
verify that we are not destroying an ABD that's still in use.  When we
are creating/destroying ABD's frequently, the performance impact of
maintaining these data structures (in particular the atomic
increment/decrement operations) can be measurable.

This commit removes this verification code on production builds, but
keeps it when ZFS_DEBUG is set.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #11535

include/sys/abd.h

@@ -52,8 +52,10 @@ typedef struct abd {
 	abd_flags_t	abd_flags;
 	uint_t		abd_size;	/* excludes scattered abd_offset */
 	list_node_t	abd_gang_link;
+#ifdef ZFS_DEBUG
 	struct abd	*abd_parent;
 	zfs_refcount_t	abd_children;
+#endif
 	kmutex_t	abd_mtx;
 	union {
 		struct abd_scatter {