LU-13997 tests: fix sanity test_418 lock cancellation

LU-13997 tests: fix sanity test_418 lock cancellation

Use "do_nodes" directly to cancel DLM locks, rather than
"do_rpc_nodes", since that is very heavy to use in a loop
(each call takes 3s, but the loop delay is only 0.2s).

Due to DoM reserving grant space for the DoM files, the "avail"
space shown by "df" may be smaller in the aggregate returned by
the MDT compared to the individual values from "lfs df".

Skip this part of the check until MDC grant cancel is fixed.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I83d989688ce671f0ff9c62ebdf3144746a3ebbe5
Reviewed-on: https://review.whamcloud.com/45231
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>