i386: remove kprobes' restore_interrupts in favour of conditional_sti

x86_64 uses a helper function conditional_sti in traps_64.c which
is equal to restore_interrupts in kprobes.h. The only user of
restore_interrupts is in traps_32.c. Introduce conditional_sti
for i386 and remove restore_interrupts.

Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
Signed-off-by: Ingo Molnar <mingo@elte.hu>