ext4: Avoid double dirtying of super block in ext4_put_super()

While reading code I noticed that ext4_put_super() dirties the 
superblock bh twice. It is always done in ext4_commit_super()
too. Remove the redundant dirty operation.
Should be a nop semantically.

Signed-off-by: Andi Kleen <ak@linux.intel.com>