powerpc/kprobes: Fix handling of instruction emulation on probe re-entry

Commit 22d8b3de ("powerpc/kprobes: Emulate instructions on kprobe
handler re-entry") enabled emulating instructions on kprobe re-entry,
rather than single-stepping always. However, we didn't update the single
stepping code to only be run if the emulation fails. Also, we missed
re-enabling preemption if the instruction emulation was successful. Fix
those issues.

Fixes: 22d8b3de ("powerpc/kprobes: Emulate instructions on kprobe handler re-entry")
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>