ext4: handle add_system_zone() failure in ext4_setup_system_zone()

There's one place that fails to handle error from add_system_zone() call
and thus we can fail to protect superblock and group-descriptor blocks
properly in case of ENOMEM. Fix it.

Reported-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20200728130437.7804-6-jack@suse.cz


Signed-off-by: Theodore Ts'o <tytso@mit.edu>