lib: sbi_scratch: zero out scratch memory on all harts

In sbi_scratch_init() we determine the last hart. The index of the last
hart cannot exceed SBI_HARTMASK_MAX_BITS - 1. We should not initialize
last_hartid_having_scratch to a higher number to avoid buffer overflows
when using this value before calling sbi_scratch_init().

When allocating scratch memory in sbi_scratch_alloc_offset() we zero out
the allocated memory for all harts except for the last one. We should not
skip the last hart.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Anup Patel <anup.patel@wdc.com>