Linux: Never sleep in kmem_cache_alloc(..., KM_NOSLEEP)

When a kmem cache is exhausted and needed to be expanded a new
slab is allocated.  KM_SLEEP callers can block and wait for the
allocation, but KM_NOSLEEP callers were incorrectly allowed to
block as well.

Resolve this by attempting an emergency allocation as a best
effort.  This may fail but that's fine since any KM_NOSLEEP
consumer is required to handle an allocation failure.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/os/linux/spl/spl-kmem-cache.c

@@ -1018,6 +1018,14 @@ spl_cache_grow(spl_kmem_cache_t *skc, int flags, void **obj)
 	might_sleep();
 	*obj = NULL;
 
+	/*
+	 * Since we can't sleep attempt an emergency allocation to satisfy
+	 * the request.  The only alterative is to fail the allocation but
+	 * its preferable try.  The use of KM_NOSLEEP is expected to be rare.
+	 */
+	if (flags & KM_NOSLEEP)
+		return (spl_emergency_alloc(skc, flags, obj));
+
 	/*
 	 * Before allocating a new slab wait for any reaping to complete and
 	 * then return so the local magazine can be rechecked for new objects.