x86: remove address space overrides using set_fs()

x86: remove address space overrides using set_fs()

Stop providing the possibility to override the address space using
set_fs() now that there is no need for that any more.  To properly
handle the TASK_SIZE_MAX checking for 4 vs 5-level page tables on
x86 a new alternative is introduced, which just like the one in
entry_64.S has to use the hardcoded virtual address bits to escape
the fact that TASK_SIZE_MAX isn't actually a constant when 5-level
page tables are enabled.

Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Signed-off-by: Al Viro <[email protected]>