lib/cmdline: Export next_arg() for being used in modules

mainline inclusion
from mainline-v5.13-rc1
commit 65dd36a3
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I7TWVA
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=65dd36a39d3b350dc96d8324b348f0863d76404d



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At least one module will benefit from using next_arg() helper.
Let's export it for that module and others if they consider it
helpful.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>

Conflicts:
	lib/cmdline.c

Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>