ext4, dax: add ext4_bmap to ext4_dax_aops

Ext4 mount path calls .bmap to the journal inode. This currently
works for the DAX mount case because ext4_iget() always set
'ext4_da_aops' to any regular files.

In preparation to fix ext4_iget() to set 'ext4_dax_aops' for ext4
DAX files, add ext4_bmap() to 'ext4_dax_aops', since bmap works for
DAX inodes.

Fixes: 5f0663bb ("ext4, dax: introduce ext4_dax_aops")
Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Suggested-by: Jan Kara <jack@suse.cz>
Cc: stable@vger.kernel.org