Squashfs: optimise squashfs_cache_get entry search

squashfs_cache_get() iterates over all entries to search for
block its looking for. Often get() / put() are called for
same block.

If we cache the current entry index, then we can optimise the
subsequent *_get() calls.

Signed-off-by: Ajeet Yadav <[e-mail]>
Signed-off-by: Phillip Lougher <[e-mail]>

fs/squashfs/cache.c

···
 	spin_lock(&cache->lock);
 
 	while (1) {
-		for (i = 0; i < cache->entries; i++)
-			if (cache->entry[i].block == block)
+		for (i = cache->curr_blk, n = 0; n < cache->entries; n++) {
+			if (cache->entry[i].block == block) {
+				cache->curr_blk = i;
 				break;
+			}
+			i = (i + 1) % cache->entries;
+		}
 
-		if (i == cache->entries) {
+		if (n == cache->entries) {
 			/*
 			 * Block not in cache, if all cache entries are used
 			 * go to sleep waiting for one to become available.
···
 		goto cleanup;
 	}
 
+	cache->curr_blk = 0;
 	cache->next_blk = 0;
 	cache->unused = entries;
 	cache->entries = entries;

fs/squashfs/squashfs_fs_sb.h

···
 struct squashfs_cache {
 	char			*name;
 	int			entries;
+	int			curr_blk;
 	int			next_blk;
 	int			num_waiters;
 	int			unused;