checkpatch: warn about unexpectedly long msleep's

As explained in Documentation/timers/timers-howto.txt, msleep's of < 20ms
may sleep for as long as 20ms.  Caller's of msleep(1) or msleep(2), etc
are likely not to expect this quirky behavior - warn them.

Signed-off-by: Patrick Pannuto <ppannuto@codeaurora.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>