arch/um: fix kunmap_atomic() call in skas/uaccess.c

kunmap_atomic() takes a pointer to within the page, not the struct page.

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>