powerpc/kprobes: Fixes for kprobe_lookup_name() on BE

Fix two issues with kprobes.h on BE which were exposed with the
optprobes work:
  - one, having to do with a missing include for linux/module.h for
    MODULE_NAME_LEN -- this didn't show up previously since the only
    users of kprobe_lookup_name were in kprobes.c, which included
    linux/module.h through other headers, and
  - two, with a missing const qualifier for a local variable which ends
    up referring a string literal. Again, this is unique to how
    kprobe_lookup_name is being invoked in optprobes.c

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>