LU-14037 osd: track commit cb in flight

LU-14037 osd: track commit cb in flight

and wait for completion in osd-ldiskfs as mntput() is not synchronous
and returns quickly scheduling real umount to another thread while
osd is shutting down. as a result commit callbacks may come when
osd has been released yet.
the patch replaces lu_device_get() and lu_device_put() for transactions
with a dedicated counter which later used to wait for callback completion.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Iaa6089b9158bc2a4bea8e33f4cfcce27395689ca
Reviewed-on: https://review.whamcloud.com/40258
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>