LU-10467 ldlm: convert waiting in ldlm_flock_completion_ast()

The l_wait_event() call in ldlm_flock_completion_ast() sets no
timeout, and so always enables fatal signals.  So it can be converted
to l_wait_event_abortable().

It is passed an on_signal handler, so that needs to be called if
l_wait_event_abortable() returns a negative result.  As this is the
only place the handler is call, it can be inlined.  We already have an
'if' which captures the 'wait was interrupted' condition, so place the
signal handler code in there.

This makes struct ldlm_flock_wait_data redundant.  In fact the
fwd_genertion field in there was already unused.

Signed-off-by: Mr NeilBrown <neilb@suse.com>
Change-Id: I9cc3a3e8b593a66f46183584382dc13169ff9adf
Reviewed-on: https://review.whamcloud.com/35984
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>