open sets ext4_da_aops for DAX existing files

["Kani, Toshi" <toshi.kani@xxxxxxx>]

I noticed that both ext4_da_aops and ext4_dax_aops are used on DAX
mounted ext4 files.  Looking at open() path:

New file
--------
  lookup_open
    ext4_create
      __ext4_new_inode
        ext4_set_inode_flags   // Set S_DAX flag
      ext4_set_aops            // Set aops to ext4_dax_aops

Existing file
-------------
  lookup_open
    ext4_lookup
      ext4_iget
        ext4_set_aops          // Set aops to ext4_da_aops
        ext4_set_inode_flags   // Set S_DAX flag

So, we set ext4_da_aops for existing files since S_DAX flag is set after
ext4_set_aops().

Thanks,
-Toshi