LU-10181 mdt: read on open for DoM files

LU-10181 mdt: read on open for DoM files

Read file data upon open and return it in reply. That works
only for file with Data-on-MDT layout and no OST components
initialized. There are three possible cases may occur:
1) file data fits in already allocated reply buffer (~9K)
   and is returned in that buffer in OPEN reply.
2) File fits in the maximum reply buffer (128K) and reply is
   returned with larger size to the client causing resend
   with re-allocated buffer.
3) File doesn't fit in reply buffer but its tail fills page
   partially then that tail is returned. This can be useful
   for an append case

Test-Parameters: mdssizegb=20 testlist=sanity-dom,dom-performance,racer
Change-Id: I5574ce5f74017fc654715e212b71fc3b905bdcae
Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Reviewed-on: https://review.whamcloud.com/23011
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>