From: Mitchell Blank Jr <mitch@sfgoth.com>

This patch makes the following improvements to might_sleep():

 o Add a "might_sleep_if()" macro for when we might sleep only if some
   condition is met.  It's a bit tidier, and has an unlikely() in it.

 o Add might_sleep checks to skb_share_check() and skb_unshare() which
   sometimes need to allocate memory.

 o Make all architectures call might_sleep() in both down() and
   down_interruptible().  Before only ppc, ppc64, and i386 did this check.
   (sh did the check on down() but not down_interruptible())