init/main.c: Fix potential static_command_line memory overflow

We allocate memory of size 'xlen + strlen(boot_command_line) + 1' for
static_command_line, but the strings copied into static_command_line are
extra_command_line and command_line, rather than extra_command_line and
boot_command_line.

When strlen(command_line) > strlen(boot_command_line), static_command_line
will overflow.

This patch just recovers strlen(command_line) which was miss-consolidated
with strlen(boot_command_line) in the commit f5c7310a ("init/main: add
checks for the return value of memblock_alloc*()")

Link: https://lore.kernel.org/all/20240412081733.35925-2-ytcoode@gmail.com/

Fixes: f5c7310a

 ("init/main: add checks for the return value of memblock_alloc*()")
Cc: stable@vger.kernel.org
Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>