This patch fixes the handling of IOPL when sysenter is used.

Currently when entering kernel mode, IOPL is not changed and it is not
presserved across context switches: thus, in the kernel, the IOPL value
is random.

This is not a problem when using iret, because it restores eflags, but
the sysexit code currently doesn't, which means that that IOPL becomes
random in user mode too which is of course not good.

This patch fixes the problem by saving eflags across context switches.