exofs: Fix CRASH on very early IO errors.

If at exofs_fill_super() we had an early termination
do to any error, like an IO error while reading the
super-block. We would crash inside exofs_free_sbi().

This is because sbi->oc.numdevs was set to 1, before
we actually have a device table at all.

Fix it by moving the sbi->oc.numdevs = 1 to after the
allocation of the device table.

Reported-by: Johannes Schild <JSchild@gmx.de>

Stable: This is a bug since v3.2.0
CC: Stable Tree <stable@kernel.org>
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>