Disable interrupts after using pmap_enter() to add the identity mapping.

Disable interrupts after using pmap_enter() to add the identity mapping.
Since pmap_enter() calls pmap_invalidate_page(), which needs interrupts
enabled in the SMP case, we defer the disable to right before saving the
register context.  This has been incorrect for about a year but caused no
real problems because the identity page never actually replaces a previously
mapped page and suspend/resume on SMP systems has been uncommon.

Tested by:	sos
MFC after:	3 days