bootconfig: Fix the kerneldoc of _xbc_exit()

Fix the kerneldoc of _xbc_exit() which is updated to have an @early
argument and the function name is changed.

Link: https://lore.kernel.org/all/171321744474.599864.13532445969528690358.stgit@devnote2/



Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202404150036.kPJ3HEFA-lkp@intel.com/
Fixes: 89f9a1e8

 ("bootconfig: use memblock_free_late to free xbc memory to buddy")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>