x86/segment: Remove .fixup usage

Create and use EX_TYPE_ZERO_REG to clear the register and retry the
segment load on exception.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20211110101325.663529463@infradead.org