For an appending write, ext3_commit_write() will call the expensive
ext3_mark_inode_dirty() twice.  Once in generic_commit_write()'s extension of
i_size and once in ext3_commit_write() itself where i_disksize is updated.

But by updating i_disksize _before_ calling generic_commit_write() these can
be piggybacked.

The patch takes the overhead of a write() from 1.96 microseconds down to
1.63.