ocfs2: pass struct file* to ocfs2_prepare_inode_for_write.

struct file * has file_ra_state to store the readahead state
and data. So pass this to ocfs2_prepare_inode_for_write. so
that it can be used in ocfs2_refcount_cow.

Signed-off-by: Tao Ma <tao.ma@oracle.com>