ext2, dax: set ext2_dax_aops for dax files

Sync syscall to DAX file needs to flush processor cache, but it
currently does not flush to existing DAX files.  This is because
'ext2_da_aops' is set to address_space_operations of existing DAX
files, instead of 'ext2_dax_aops', since S_DAX flag is set after
ext2_set_aops() in the open path.

Similar to ext4, change ext2_iget() to initialize i_flags before
ext2_set_aops().

Fixes: fb094c90

 ("ext2, dax: introduce ext2_dax_aops")
Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
Suggested-by: Jan Kara <jack@suse.cz>
Cc: Jan Kara <jack@suse.cz>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: <stable@vger.kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>