perf probe: Fix offset to allow signed value

Fix dereference offset to intmax_t from uintmax_t, because
it can have negative values (for example local variable's offset
from frame pointer).

Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
Cc: systemtap <systemtap@sources.redhat.com>
Cc: DLE <dle-develop@lists.sourceforge.net>
LKML-Reference: <20100315170228.31852.71946.stgit@localhost6.localdomain6>
Signed-off-by: Ingo Molnar <mingo@elte.hu>