mdopen: call "modprobe md_mod" if it might be needed.

mdopen: call "modprobe md_mod" if it might be needed.

Creating an array by opening a block-device with major number of 9
will transparently load the md module if needed.
Creating an array by opening
   /sys/module/md_mod/parameters/new_array
and writing to it won't, it will just fail if md_mod isn't loaded.

So when opening that file fails with ENOENT, run "modprobe md_mod" and
try again.

This fixes a bug whereby if you have "CREATE names=yes" in mdadm.conf,
and the md modules isn't loaded, then creating or assembling an
array will not honor the "names=yes" configuration.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>