x86/ia32: Restore r8 correctly in 32bit SYSCALL instruction entry.

x86/ia32: Restore r8 correctly in 32bit SYSCALL instruction entry.

This commit fixes a bug in a previous commit
8e69671028ac ("x86/ia32: Adds code hygiene for 32bit SYSCALL instruction
entry.")

SAVE_EXTRA_REGS does not save the r8 register. r8 is rather saved in
pt_regs->sp before it is cleared. So, retrieve r8 from pt_regs->sp.

Orabug: 28529706

Signed-off-by: Gayatri Vasudevan <gayatri.vasudevan@oracle.com>
Reviewed-by: Bert Barbe <bert.barbe@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>