Ah. Don't use NENTRY() to declare check_swapgs, use LABEL() instead. NENTRY
puts the code in the .text section, so the effect of TEXT_USER_BEGIN was
overwritten, and check_swapgs was not put in the .text.user section.
As a result kernels running SVS would crash when jumping here - because we
execute this place with the user page table loaded, and in this page table
only .text.user is mapped.
While here, rename check_swapgs -> kernuser_reenter, because we do more
things than just SWAPGS.
UnifiedSplitRaw
