- iget5_locked in bdget really needs blockdev_superblock, instead of
  bd_mnt, so bd_mnt could be just a local variable;

- blockdev_superblock really needs __read_mostly, while local var bd_mnt
  not;

- make use of sb_is_blkdev_sb in bd_forget, instead of direct reference
  to blockdev_superblock.
Signed-off-by: NDenis ChengRq <crquan@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>