tracing/probes: Move 'symbol' fetch method to kprobes

Move existing functions to trace_kprobe.c and add NULL entries to the
uprobes fetch type table.  I don't make them static since some generic
routines like update/free_XXX_fetch_param() require pointers to the
functions.

Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: zhangwei(Jovi) <jovi.zhangwei@huawei.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>