lib/bootconfig: change xbc_node_index() return type to uint16_t

  lib/bootconfig.c:136:21: warning: conversion from 'long int' to
  'int' may change value [-Wconversion]
  lib/bootconfig.c:308:33: warning: conversion from 'int' to 'uint16_t'
  may change value [-Wconversion]
  lib/bootconfig.c:467:37: warning: conversion from 'int' to 'uint16_t'
  may change value [-Wconversion]
  lib/bootconfig.c:469:40: warning: conversion from 'int' to 'uint16_t'
  may change value [-Wconversion]
  lib/bootconfig.c:472:54: warning: conversion from 'int' to 'uint16_t'
  may change value [-Wconversion]
  lib/bootconfig.c:476:45: warning: conversion from 'int' to 'uint16_t'
  may change value [-Wconversion]

xbc_node_index() returns the position of a node in the xbc_nodes array,
which has at most XBC_NODE_MAX (8192) entries, well within uint16_t
range.  Every caller stores the result in a uint16_t field (node->parent,
node->child, node->next, or the keys[] array in compose_key_after), so
the int return type causes narrowing warnings at all six call sites.

Change the return type to uint16_t and add an explicit cast on the
pointer subtraction to match the storage width and eliminate the
warnings.

Link: https://lore.kernel.org/all/20260318155919.78168-14-objecting@objecting.org/

Signed-off-by: Josh Law <objecting@objecting.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>