ext4: Use readahead when reading an inode from the inode table

With modern hard drives, reading 64k takes roughly the same time as
reading a 4k block.  So request readahead for adjacent inode table
blocks to reduce the time it takes when iterating over directories
(especially when doing this in htree sort order) in a cold cache case.
With this patch, the time it takes to run "git status" on a kernel
tree after flushing the caches via "echo 3 > /proc/sys/vm/drop_caches"
is reduced by 21%.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/ext4_sb.h

@@ -52,6 +52,7 @@ struct ext4_sb_info {
 	int s_desc_per_block_bits;
 	int s_inode_size;
 	int s_first_ino;
+	unsigned int s_inode_readahead_blks;
 	spinlock_t s_next_gen_lock;
 	u32 s_next_generation;
 	u32 s_hash_seed[4];