lib/bootconfig: use signed type for offset in xbc_init_node()

lib/bootconfig: use signed type for offset in xbc_init_node()

  lib/bootconfig.c:415:32: warning: conversion to 'long unsigned int'
  from 'long int' may change the sign of the result [-Wsign-conversion]

Pointer subtraction yields ptrdiff_t (signed), which was stored in
unsigned long.  The original unsigned type implicitly caught a negative
offset (data < xbc_data) because the wrapped value would exceed
XBC_DATA_MAX.  Make this intent explicit by using a signed long and
adding an offset < 0 check to the WARN_ON condition.

Link: https://lore.kernel.org/all/20260318155919.78168-12-objecting@objecting.org/
Signed-off-by: Josh Law <objecting@objecting.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>