LU-12343 osc: Fix dom handling in weight_ast

LU-12343 osc: Fix dom handling in weight_ast

The DOM bit can be cancelled at any time during calls to
weigh_ast, so:

1. We cannot assert that it is present
2. We cannot use it to identify the !LDLM_EXTENT case when
calling osc_lock_weight

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ic3e7370580e35d8ae06b8330971959e0d55a4e81
Reviewed-on: https://review.whamcloud.com/34966
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>