LU-10894 dom: mdc_lock_flush() improvement

LU-10894 dom: mdc_lock_flush() improvement

There is small improvement in osc_lock_flush() to don't
match other locks for write lock because there are none.

Do the same in mdc_lock_flush().

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: Ie18ef63b2f969f762f0263f8b93aea726f89305f
Reviewed-on: https://review.whamcloud.com/34738
Tested-by: Jenkins
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andrew Perepechko <c17827@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>