zfsboot: Fix startup crash

zfsboot: Fix startup crash

On a FreeNAS mini XL, with geli encrypted drives the loader crashed in
geli_read().

When we iterate over the list of disks and allocate the zfsdsk structures we
don’t zero out the gdev pointer. In one case that resulted in geli_read()
(called on the bogus pointer) dividing by zero.

Use calloc() to ensure the zfsdsk structure is always zeroed, so the pointer is
initialised to NULL. As a side benefit it gets rid of one #ifdef
LOADER_GELI_SUPPORT.