watchdog: handle comma separated nmi_watchdog command line

Per the document, the kernel can accept comma separated command line like
nmi_watchdog=nopanic,0.  However, the code doesn't really handle it.  Fix
the kernel to handle it properly.

Link: https://lkml.kernel.org/r/20240430060236.1878002-1-song@kernel.org


Signed-off-by: Song Liu <song@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>