fs: ext2: Fix duplicate included linux/dax.h

Clean up the following includecheck warning:

fs/ext2/inode.c: linux/dax.h is included more than once.

No functional change.

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/1648008123-32485-1-git-send-email-baihaowen@meizu.com