Okay, this is the final version of the sd patch to remove sd_dsk_arr.
Instead of keeping a big global array we allocate one structure for
each disk dynamically in sd_attach.  We can use the higher level
private data to access it almost everywhere, for the few other
places we keep a linked list of all disks (these are walked only
in slow-patheß).

This patch also allows to get rid of the sd_init midlayer method and
the CONFIG_SD_EXTRA_DEVS option - sd has no more static limit to
the number of disk except of the number of assigned majors.