LU-13424 mdt: fix DOM entry check

LU-13424 mdt: fix DOM entry check

The mdt_lmm_dom_entry() tries to determine if layout has
DOM entry or not but does that correctly only if DOM entry
is the first one in layout and file is not mirrored.
Otherwise it scans through all entries and wronly reports
that DOM entry exists.

Patch does the following:
- correct initialization of 'has_dom=false' by default
- correct check for DOM+OST case, only if both entries were
  found among initiated entries
- add test case for directory migration with mirrored file

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: I43c33528213132f03ebec2debec48b8d53ee229f
Reviewed-on: https://review.whamcloud.com/38257
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>