Initialize mnt_stat.f_iosize before autostarting UFS1 extattrs.

It is normally initialized by ffs_statfs() after ffs_mount finished.

The extattr autostart code calls the ufs_lookup(), that uses value above
to iterate over the directory blocks, see bmask initialization in the
ufs_lookup() and ufsdirhash. Having the filesystem with root directory
spanning more then one block would result in reading a random kernel
memory.

PR:	kern/120781
Test case provided by:	rwatson
MFC after:	1 week

sys/ufs/ffs/ffs_vfsops.c

@@ -880,6 +880,7 @@ ffs_mountfs(devvp, mp, td)
 	 * This would all happen while the filesystem was busy/not
 	 * available, so would effectively be "atomic".
 	 */
+	mp->mnt_stat.f_iosize = fs->fs_bsize;
 	(void) ufs_extattr_autostart(mp, td);
 #endif /* !UFS_EXTATTR_AUTOSTART */
 #endif /* !UFS_EXTATTR */