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>