nfsd: fix possible read-ahead cache and export table corruption

The value of nperbucket calculated here is too small--we should be rounding up
instead of down--with the result that the index j in the following loop can
overflow the raparm_hash array.  At least in my case, the next thing in memory
turns out to be export_table, so the symptoms I see are crashes caused by the
appearance of four zeroed-out export entries in the first bucket of the hash
table of exports (which were actually entries in the readahead cache, a
pointer to which had been written to the export table in this initialization
code).

It looks like the bug was probably introduced with commit
fce1456a19f5c08b688c29f00ef90fdfa074c79b ("knfsd: make the readahead params
cache SMP-friendly").

Cc: <stable@kernel.org>
Cc: Greg Banks <gnb@melbourne.sgi.com>
Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Acked-by: NeilBrown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/nfsd/vfs.c

@@ -1916,7 +1916,7 @@ nfsd_racache_init(int cache_size)
 		raparm_hash[i].pb_head = NULL;
 		spin_lock_init(&raparm_hash[i].pb_lock);
 	}
-	nperbucket = cache_size >> RAPARM_HASH_BITS;
+	nperbucket = DIV_ROUND_UP(cache_size, RAPARM_HASH_SIZE);
 	for (i = 0; i < cache_size - 1; i++) {
 		if (i % nperbucket == 0)
 			raparm_hash[j++].pb_head = raparml + i;