Add .va_dentry helper

While this extra structure memory does not exist under Solaris
it is needed under Linux to pass the dentry.  This allows the
dentry to be easily instantiated before the inode is unlocked.

include/sys/vnode.h

@@ -140,6 +140,7 @@ typedef struct vattr {
 	uint64_t	va_nblocks;	/* space used */
 	uint32_t	va_blksize;	/* block size */
 	uint32_t	va_seq;		/* sequence */
+	struct dentry	*va_dentry;	/* dentry to wire */
 } vattr_t;
 
 typedef struct vnode {