ftrace: Initialize modname and offset in print_rec()

print_rec() leaves 'modname' and 'offset' uninitialized and
kallsyms_lookup() only sets them on success. When the lookup fails like
when a module is being loaded or unloaded, 'modname' holds a garbage
value, passes the 'if (modname)' check, and is dereferenced by
seq_printf().

  Oops: general protection fault
  RIP: 0010:string+0x48/0xe0
  Call Trace:
   vsnprintf+0x1f5/0x650
   seq_printf+0x8e/0xb0
   print_rec+0x6c/0xc0
   t_show+0x4c/0x440
   seq_read_iter+0x119/0x480

This can be triggered by reading available_filter_functions while a
module is being loaded or unloaded.

Initialize both variables so a failed lookup skips the module name.

Cc: stable@vger.kernel.org
Fixes: b39181f7c690 ("ftrace: Add FTRACE_MCOUNT_MAX_OFFSET to avoid adding weak function")
Link: https://patch.msgid.link/20261016230403.4916-1-aarnahmd@amazon.com
Tested-by: Gavin Inglis <giinglis@amazon.com>
Signed-off-by: Aaron Ahmed <aarnahmd@amazon.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>