Kprobes: Make kprobe.symbol_name const

Kprobes doesn't scribble the kprobe.symbol_name field.  Its only set by the
module when registering the probe.  Modules that exercise good hygiene
using the "const" qualifier will see warnings...

	warning: assignment discards qualifiers from pointer target type

Make struct kprobe.symbol_name const char *

Signed-off-by: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Signed-off-by: Jim Keniston <jkenisto@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>