Implement the might_sleep_if() function macro in the LinuxKPI.

Submitted by:	Johannes Lundberg <johalun0@gmail.com>
MFC after:	1 week
Sponsored by:	Mellanox Technologies
Sponsored by:	Limelight Networks

sys/compat/linuxkpi/common/include/linux/wait.h

@@ -47,6 +47,10 @@
 #define	might_sleep()							\
 	WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL, "might_sleep()")
 
+#define	might_sleep_if(cond) do { \
+	if (cond) { might_sleep(); } \
+} while (0)
+
 struct wait_queue;
 struct wait_queue_head;