x86/ioapic: Fix fixmap error path logic in ioapic_init_mappings()

x86/ioapic: Fix fixmap error path logic in ioapic_init_mappings()

In the case that bad_ioapic_register() fails, the current position of idx++
means that clear_fixmap(idx) will be called with the wrong index, and not
clean up the mapping just created.

Increment idx as part of the loop, rather than midway through the loop body.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>