ext4: auto disable dax instead of failing mount

Bring the ext4 filesystem in line with xfs that only warns and continues
when the "-o dax" option is specified to mount and the backing device
does not support dax. This is in preparation for removing dax support
from devices that do not enable get_user_pages() operations on dax
mappings. In other words 'gup' support is required and configurations
that were using so called 'page-less' dax will be converted back to
using the page cache.

Removing the broken 'page-less' dax support is a pre-requisite for
removing the "EXPERIMENTAL" warning when mounting a filesystem in dax
mode.
Reviewed-by: NJan Kara <jack@suse.cz>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

fs/ext4/super.c

@@ -3710,11 +3710,14 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 		if (ext4_has_feature_inline_data(sb)) {
 			ext4_msg(sb, KERN_ERR, "Cannot use DAX on a filesystem"
 					" that may contain inline data");
-			goto failed_mount;
+			sbi->s_mount_opt &= ~EXT4_MOUNT_DAX;
 		}
 		err = bdev_dax_supported(sb, blocksize);
-		if (err)
-			goto failed_mount;
+		if (err) {
+			ext4_msg(sb, KERN_ERR,
+				"DAX unsupported by block device. Turning off DAX.");
+			sbi->s_mount_opt &= ~EXT4_MOUNT_DAX;
+		}
 	}
 
 	if (ext4_has_feature_encrypt(sb) && es->s_encryption_level) {