Fix link count of root inode when snapdir is visible

Given how zfs_getattr() is implemented, zfs_getattr_fast() (used by
->getattr() of zpl inodes) also needs to consider an additional link
count if "snapdir" property is set to "visible".

Without this, # of directories in root inode of each dataset doesn't
match the link count when snapdir is visible.

Reviewed-by: Richard Yao <ryao@gentoo.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@osnexus.com>
Closes #8727

module/zfs/zfs_vnops.c

@@ -2657,6 +2657,12 @@ zfs_getattr_fast(struct inode *ip, struct kstat *sp)
 	mutex_enter(&zp->z_lock);
 
 	generic_fillattr(ip, sp);
+	/*
+	 * +1 link count for root inode with visible '.zfs' directory.
+	 */
+	if ((zp->z_id == zfsvfs->z_root) && zfs_show_ctldir(zp))
+		if (sp->nlink < ZFS_LINK_MAX)
+			sp->nlink++;
 
 	sa_object_size(zp->z_sa_hdl, &blksize, &nblocks);
 	sp->blksize = blksize;