nfsd4: check for uninitialized slot

This fixes an oops when a buggy client tries to use an initial seqid of
0 on a new slot, which we may misinterpret as a replay.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
diff --git a/fs/nfsd/state.h b/fs/nfsd/state.h
index 122217f..d8f52a0 100644
--- a/fs/nfsd/state.h
+++ b/fs/nfsd/state.h
@@ -134,6 +134,7 @@
 	u16	sl_opcnt;
 #define NFSD4_SLOT_INUSE	(1 << 0)
 #define NFSD4_SLOT_CACHETHIS	(1 << 1)
+#define NFSD4_SLOT_INITIALIZED	(1 << 2)
 	u8	sl_flags;
 	char	sl_data[];
 };