LU-2047 obdfilter: init glimpse desc to NULL for ext lock

LU-2047 obdfilter: init glimpse desc to NULL for ext lock

There is no glimpse descriptor when glimpsing extent locks, we should
thus set ldlm_glimpse_work::gl_desc to NULL to allocate glimpse
request with the correct format.

Signed-off-by: Johann Lombardi <johann@whamcloud.com>
Change-Id: I5cdedfb79631328114ef7b0bc8e035dde985b4fa
Reviewed-on: http://review.whamcloud.com/4127
Tested-by: Hudson
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>