LU-12003 osd: take reference to object in osd_trunc_lock()

LU-12003 osd: take reference to object in osd_trunc_lock()

normally the references to objects are held until a transaction
is over, but in few cases reference is released before. and then
such an object can be release, so OSD should have own reference
to prevent early release.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I81647fdec8d42f123e990553edb5e371636f45c0
Reviewed-on: https://review.whamcloud.com/37170
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>