dm cache: display 'needs_check' in status if it is set

There is currently no way to see that the needs_check flag has been set
in the metadata.  Display 'needs_check' in the cache status if it is set
in the cache metadata.

Also, update cache documentation.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
diff --git a/Documentation/device-mapper/cache.txt b/Documentation/device-mapper/cache.txt
index 82960cf..785eab8 100644
--- a/Documentation/device-mapper/cache.txt
+++ b/Documentation/device-mapper/cache.txt
@@ -258,6 +258,12 @@
 	no further I/O will be permitted and the status will just
 	contain the string 'Fail'.  The userspace recovery tools
 	should then be used.
+needs_check		 : 'needs_check' if set, '-' if not set
+	A metadata operation has failed, resulting in the needs_check
+	flag being set in the metadata's superblock.  The metadata
+	device must be deactivated and checked/repaired before the
+	cache can be made fully operational again.  '-' indicates
+	needs_check is not set.
 
 Messages
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