nilfs2: fix sync silent failure

As of 32a88aa1, __sync_filesystem() will return 0 if s_bdi is not set.
And nilfs does not set s_bdi anywhere.  I noticed this problem by the
warning introduced by the recent commit 5129a469 ("Catch filesystem
lacking s_bdi").

 WARNING: at fs/super.c:959 vfs_kern_mount+0xc5/0x14e()
 Hardware name: PowerEdge 2850
 Modules linked in: nilfs2 loop tpm_tis tpm tpm_bios video shpchp pci_hotplug output dcdbas
 Pid: 3773, comm: mount.nilfs2 Not tainted 2.6.34-rc6-debug #38
 Call Trace:
  [<c1028422>] warn_slowpath_common+0x60/0x90
  [<c102845f>] warn_slowpath_null+0xd/0x10
  [<c1095936>] vfs_kern_mount+0xc5/0x14e
  [<c1095a03>] do_kern_mount+0x32/0xbd
  [<c10a811e>] do_mount+0x671/0x6d0
  [<c1073794>] ? __get_free_pages+0x1f/0x21
  [<c10a684f>] ? copy_mount_options+0x2b/0xe2
  [<c107b634>] ? strndup_user+0x48/0x67
  [<c10a81de>] sys_mount+0x61/0x8f
  [<c100280c>] sysenter_do_call+0x12/0x32

This ensures to set s_bdi for nilfs and fixes the sync silent failure.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>