bootconfig: init: Fix memblock leak in xbc_make_cmdline()

Free unused memblock in a error case to fix memblock leak
in xbc_make_cmdline().

Link: https://lkml.kernel.org/r/163177339181.682366.8713781325929549256.stgit@devnote2



Fixes: 51887d03 ("bootconfig: init: Allow admin to use bootconfig for kernel command line")
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>