Fix -Wattribute-warning in zfs_log_xvattr()

Restructure the code in zfs_log_xvattr() to use a lr_attr_end
structure when accessing lr_attr_t elements located after the
variable sized array.  This makes the code more understandable
and resolves the accessing beyond the end of the field warnings.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13528
Closes #13575

include/sys/zil.h

@@ -221,6 +221,15 @@ typedef struct {
 	uint64_t	lr_foid;	/* object id */
 } lr_ooo_t;
 
+/*
+ * Additional lr_attr_t fields.
+ */
+typedef struct {
+	uint64_t	lr_attr_attrs;		/* all of the attributes */
+	uint64_t	lr_attr_crtime[2];	/* create time */
+	uint8_t		lr_attr_scanstamp[32];
+} lr_attr_end_t;
+
 /*
  * Handle option extended vattr attributes.
  *
@@ -231,7 +240,7 @@ typedef struct {
 typedef struct {
 	uint32_t	lr_attr_masksize; /* number of elements in array */
 	uint32_t	lr_attr_bitmap; /* First entry of array */
-	/* remainder of array and any additional fields */
+	/* remainder of array and additional lr_attr_end_t fields */
 } lr_attr_t;
 
 /*