block/keyslot-manager: prevent crash when num_slots=1

If there is only one keyslot, then blk_ksm_init() computes
slot_hashtable_size=1 and log_slot_ht_size=0.  This causes
blk_ksm_find_keyslot() to crash later because it uses
hash_ptr(key, log_slot_ht_size) to find the hash bucket containing the
key, and hash_ptr() doesn't support the bits == 0 case.

Fix this by making the hash table always have at least 2 buckets.

Tested by running:

    kvm-xfstests -c ext4 -g encrypt -m inlinecrypt \
                 -o blk-crypto-fallback.num_keyslots=1

Fixes: 1b262839 ("block: Keyslot Manager for Inline Encryption")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/keyslot-manager.c

@@ -103,6 +103,13 @@ int blk_ksm_init(struct blk_keyslot_manager *ksm, unsigned int num_slots)
 	spin_lock_init(&ksm->idle_slots_lock);
 
 	slot_hashtable_size = roundup_pow_of_two(num_slots);
+	/*
+	 * hash_ptr() assumes bits != 0, so ensure the hash table has at least 2
+	 * buckets.  This only makes a difference when there is only 1 keyslot.
+	 */
+	if (slot_hashtable_size < 2)
+		slot_hashtable_size = 2;
+
 	ksm->log_slot_ht_size = ilog2(slot_hashtable_size);
 	ksm->slot_hashtable = kvmalloc_array(slot_hashtable_size,
 					     sizeof(ksm->slot_hashtable[0]),