repair: convert the dir byaddr hash to a radix tree

repair: convert the dir byaddr hash to a radix tree

Phase 6 uses a hash table to track the data segment addresses of the
entries it has seen in a directory. This is indexed by the offset
into the data segment for the dirent, and is used to check if the
entry exists, is a duplicate or has a bad hash value. The lookup
operations involve walking long hash chains on large directories and
they are done for every entry in the directory. This means certain
operations have O(n^2) scalability (or worse!) and hence hurt on
very large directories.

It is also used to determine if the directory has unseen entries,
which involves a full hash traversal that is very expensive on large
directories. Hence the directory checking for unseen ends up being
roughly a O(n^2 + n) algorithm.

Switch the byaddr indexing to a radix tree. While a radix tree will
burn more memory than the linked list, it gives us O(log n) lookup
operations instead of O(n) on large directories, and use for tags
gives us O(1) determination of whether all entries have been seen or
not. This brings the "entry seen" algorithm scalability back to
O(nlog n) and so is a major improvement for processing large
directories.

Given a filesystem with 10M empty files in a single directory, we
see:

5.6.0:

  97.56%  xfs_repair              [.] dir_hash_add.lto_priv.0
   0.38%  xfs_repair              [.] avl_ino_start.lto_priv.0
   0.37%  libc-2.31.so            [.] malloc
   0.34%  xfs_repair              [.] longform_dir2_entry_check_data.lto_priv.0

Phase 6:        10/22 12:07:13  10/22 12:10:51  3 minutes, 38 seconds

Patched:

  97.11%  xfs_repair          [.] dir_hash_add
   0.38%  xfs_repair          [.] longform_dir2_entry_check_data
   0.34%  libc-2.31.so        [.] __libc_calloc
   0.32%  xfs_repair          [.] avl_ino_start

Phase 6:        10/22 12:11:40  10/22 12:14:28  2 minutes, 48 seconds

So there's some improvement, but we are clearly still CPU bound due
to the O(n^2) scalability of the duplicate name checking algorithm.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>