mdopen: always try create_named_array()

mdopen: always try create_named_array()

mdopen() will use create_named_array() to ask the kernel to create the
given md array, but only if it is given a number or name.
If it is NOT given a name and is required to choose one itself using
find_free_devnm() it does NOT use create_named_array().

On kernels with CONFIG_BLOCK_LEGACY_AUTOLOAD not set, this can result in
failure to assemble an array.  This can particularly seen when the
"name" of the array begins with a host name different to the name of the
host running the command.

So add the missing call to create_named_array().

Link: https://bugzilla.kernel.org/show_bug.cgi?id=217074
Signed-off-by: NeilBrown <neilb@suse.de>
Acked-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Signed-off-by: Jes Sorensen <jes@trained-monkey.org>