mm/vmalloc.c: improve vfree() kerneldoc

vfree() might sleep if called not in interrupt context.  Explain that in
the comment.

Link: http://lkml.kernel.org/r/20180914130512.10394-2-aryabinin@virtuozzo.com
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmalloc.c

@@ -1577,6 +1577,8 @@ void vfree_atomic(const void *addr)
  *	have CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG, but making the calling
  *	conventions for vfree() arch-depenedent would be a really bad idea)
  *
+ *	May sleep if called *not* from interrupt context.
+ *
  *	NOTE: assumes that the object at @addr has a size >= sizeof(llist_node)
  */
 void vfree(const void *addr)