Currently we limit the size of the roots array, for backref cache entries,
to 12 elements. This is because that number is enough for most cases and
to make the backref cache entry size to be exactly 128 bytes, so that
memory is allocated from the kmalloc-128 slab and no space is wasted.

However recent changes in the series refactored the backref cache to be
more generic and allow it to be reused for other purposes, which resulted
in increasing the size of the embedded structure btrfs_lru_cache_entry in
order to allow for supporting inode numbers as keys on 32 bits system and
allow multiple generations per key. This resulted in increasing the size
of struct backref_cache_entry from 128 bytes to 152 bytes. Since the cache
entries are allocated with kmalloc(), it means we end up using the slab
kmalloc-192, so we end up wasting 40 bytes of memory. So bump the size of
the roots array from 12 elements to 17 elements, so we end up using 192
bytes for each backref cache entry.

This patch is part of a larger patchset and the changelog of the last
patch in the series contains a sample performance test and results.
The patches that comprise the patchset are the following:

  btrfs: send: directly return from did_overwrite_ref() and simplify it
  btrfs: send: avoid unnecessary generation search at did_overwrite_ref()
  btrfs: send: directly return from will_overwrite_ref() and simplify it
  btrfs: send: avoid extra b+tree searches when checking reference overrides
  btrfs: send: remove send_progress argument from can_rmdir()
  btrfs: send: avoid duplicated orphan dir allocation and initialization
  btrfs: send: avoid unnecessary orphan dir rbtree search at can_rmdir()
  btrfs: send: reduce searches on parent root when checking if dir can be removed
  btrfs: send: iterate waiting dir move rbtree only once when processing refs
  btrfs: send: initialize all the red black trees earlier
  btrfs: send: genericize the backref cache to allow it to be reused
  btrfs: adapt lru cache to allow for 64 bits keys on 32 bits systems
  btrfs: send: cache information about created directories
  btrfs: allow a generation number to be associated with lru cache entries
  btrfs: add an api to delete a specific entry from the lru cache
  btrfs: send: use the lru cache to implement the name cache
  btrfs: send: update size of roots array for backref cache entries
  btrfs: send: cache utimes operations for directories if possible
Signed-off-by: NFilipe Manana <fdmanana@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>