LU-17038 tests: remove mlink utility

LU-17038 tests: remove mlink utility

The mlink utility is nearly identical to the link utility
provided by coreutils. They only differ by some GNU
boilerplate. All tests using mlink are replaced with link.
Luckily, mlink is only used in a few places.

Used the following command:

 `git grep -i mlink | grep -i -v symlink`

to track down all uses of mlink.

Test-Parameters: trivial testlist=recovery-small
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I197235572d2cb267ee68930c64058e4f5ffe5be1
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52051
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>