uml: x86_64 should copy %fs during fork

%fs needs to be copied from parent to child during fork.

Tidied up some whitespace while I was here.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>