ext4: cache all of an extent tree's leaf block upon reading

When we read in an extent tree leaf block from disk, arrange to have
all of its entries cached.  In nearly all cases the in-memory
representation will be more compact than the on-disk representation in
the buffer cache, and it allows us to get the information without
having to traverse the extent tree for successive extents.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>

fs/ext4/extents_status.h

@@ -71,6 +71,9 @@ extern void ext4_es_init_tree(struct ext4_es_tree *tree);
 extern int ext4_es_insert_extent(struct inode *inode, ext4_lblk_t lblk,
 				 ext4_lblk_t len, ext4_fsblk_t pblk,
 				 unsigned int status);
+extern void ext4_es_cache_extent(struct inode *inode, ext4_lblk_t lblk,
+				 ext4_lblk_t len, ext4_fsblk_t pblk,
+				 unsigned int status);
 extern int ext4_es_remove_extent(struct inode *inode, ext4_lblk_t lblk,
 				 ext4_lblk_t len);
 extern void ext4_es_find_delayed_extent_range(struct inode *inode,