ext4: don't complain about incorrect features when probing

If mount is auto-probing for filesystem type, it will try various
filesystems in order, with the MS_SILENT flag set.  We get
that flag as the silent arg to ext4_fill_super.

If we're probing (silent==1) then don't complain about feature
incompatibilities that are found if it looks like it's actually
a different valid extN type - failed probes should be silent
in this case.

If the on-disk features are unknown even to ext4, then complain.
Reported-by: Joakim Tjernlund <Joakim.Tjernlund@infinera.com>
Tested-by: Joakim Tjernlund <Joakim.Tjernlund@infinera.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

fs/ext4/super.c

@@ -3662,6 +3662,12 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 			ext4_msg(sb, KERN_INFO, "mounting ext2 file system "
 				 "using the ext4 subsystem");
 		else {
+			/*
+			 * If we're probing be silent, if this looks like
+			 * it's actually an ext[34] filesystem.
+			 */
+			if (silent && ext4_feature_set_ok(sb, sb_rdonly(sb)))
+				goto failed_mount;
 			ext4_msg(sb, KERN_ERR, "couldn't mount as ext2 due "
 				 "to feature incompatibilities");
 			goto failed_mount;
@@ -3673,6 +3679,12 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 			ext4_msg(sb, KERN_INFO, "mounting ext3 file system "
 				 "using the ext4 subsystem");
 		else {
+			/*
+			 * If we're probing be silent, if this looks like
+			 * it's actually an ext4 filesystem.
+			 */
+			if (silent && ext4_feature_set_ok(sb, sb_rdonly(sb)))
+				goto failed_mount;
 			ext4_msg(sb, KERN_ERR, "couldn't mount as ext3 due "
 				 "to feature incompatibilities");
 			goto failed_mount;