LU-16843 ldiskfs: merge extent blocks

LU-16843 ldiskfs: merge extent blocks

There are cases (e.g. file written synchronously with discontiguous
blocks that are later filled in) when a lot of extents are created
initially, then the extents get merged over time, but there is no
way to merge the index blocks.  This can cause a very deep extent
index tree (above 5 levels) and cause problems like:

  inode has invalid extent depth: 6

Merge leave/index blocks (one at each level at most) to right/left
when extents are removed from the index.

submitted to ext4@ maillist:
https://lore.kernel.org/linux-ext4/7A2B8861-96AA-4815-BB58-180F63F62436@whamcloud.com/

Lustre-change: https://review.whamcloud.com/51096
Lustre-commit: 0f7e6c02a9ea5b85d8d97f724bed318268cea60f

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I746c0917e746eb442d3c69a23f591d9cdade76fa
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51272
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>