LU-18500 mdc: rename mdc_get_unused

LU-18500 mdc: rename mdc_get_unused

It doesn't 'get' unused - it cancels them.

Actually, it cancels unused conflicting locks, but that
made the name too long and hopefully it's clear from the
signature.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I3f01f79a63069dfb019667ff4ca0d427e1699511
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57200
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>