LU-11420 mdt: export mdt_dom_read_open_fops

LU-11420 mdt: export mdt_dom_read_open_fops

Read-on-open MDT lproc fops were removed by mistake in
commit 169738e3, restore lost changes and fix compilation
failures with gcc8

Change-Id: I89de27b76e061b67d175d026a7e563d48df78603
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/33224
Tested-by: Jenkins
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>