ext4: add proc files to monitor new structures

ext4: add proc files to monitor new structures

This patch adds a new file "mb_structs_summary" which allows us to see
the summary of the new allocator structures added in this
series. Here's the sample output of file:

optimize_scan: 1
max_free_order_lists:
        list_order_0_groups: 0
        list_order_1_groups: 0
        list_order_2_groups: 0
        list_order_3_groups: 0
        list_order_4_groups: 0
        list_order_5_groups: 0
        list_order_6_groups: 0
        list_order_7_groups: 0
        list_order_8_groups: 0
        list_order_9_groups: 0
        list_order_10_groups: 0
        list_order_11_groups: 0
        list_order_12_groups: 0
        list_order_13_groups: 40
fragment_size_tree:
        tree_min: 16384
        tree_max: 32768
        tree_nodes: 40

Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Reviewed-by: Ritesh Harjani <ritesh.list@gmail.com>
Link: https://lore.kernel.org/r/20210401172129.189766-7-harshadshirwadkar@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>