knfsd: fix reply cache memory corruption

Fix a regression in the reply cache introduced when the code was
converted to use proper Linux lists. When a new entry needs to be
inserted, the case where all the entries are currently being used
by threads is not correctly detected. This can result in memory
corruption and a crash. In the current code this is an extremely
unlikely corner case; it would require the machine to have 1024
nfsd threads and all of them to be busy at the same time. However,
upcoming reply cache changes make this more likely; a crash due to
this problem was actually observed in field.

Signed-off-by: Greg Banks <[e-mail]>
Signed-off-by: J. Bruce Fields <[e-mail]>

fs/nfsd/nfscache.c

···
 	}
 	}
 
-	/* This should not happen */
-	if (rp == NULL) {
+	/* All entries on the LRU are in-progress. This should not happen */
+	if (&rp->c_lru == &lru_head) {
 		static int	complaints;
 
 		printk(KERN_WARNING "nfsd: all repcache entries locked!\n");