At the parsing phase of mount in the new mount api sb will not be
available so allow sb to be NULL in ext4_msg and use that in
handle_mount_opt().

Also change return value to appropriate -EINVAL where needed.
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Link: https://lore.kernel.org/r/20211027141857.33657-6-lczerner@redhat.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>