Scanning the trees just to see if there is anything to abort is
unnecessary -- all that is needed here is to update the epoch barrier
first, before we start aborting.  Simplify and do the update inside the
loop before calling abort_request() for the first time.

The switch to for_each_request() also fixes a bug: homeless requests
weren't even considered for aborting.
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>
Acked-by: NJeff Layton <jlayton@redhat.com>
Reviewed-by: N"Yan, Zheng" <zyan@redhat.com>