LU-16589 tests: fix hard-link failure in sanityn/55d

LU-16589 tests: fix hard-link failure in sanityn/55d

Since coreutils version 8.31, the stat() and lstat()
operations were removed from ln by commit 571f63f5010b,
which caused the following dir hard-link failure in
sanityn/55d:

ln: failed to create hard link '/mnt/lustre2/d55d.sanityn/d55d.sanityn/'
=> '/mnt/lustre2/d55d.sanityn/f1': No such file or directory

This actually reveals a kernel issue which is fixed by commit
v5.18-rc2-188-gb3d4650d82c7.

To avoid the kernel issue and keep the test effective,
this patch appends the target filename to the $tdir/
so as to fix the hard-link failure.

Test-Parameters: trivial env=ONLY=55d testlist=sanityn
Test-Parameters: trivial clientdistro=el9.1 env=ONLY=55d testlist=sanityn
Test-Parameters: trivial clientdistro=el9.0 env=ONLY=55d testlist=sanityn
Test-Parameters: trivial clientdistro=sles15sp4 env=ONLY=55d testlist=sanityn
Test-Parameters: trivial clientdistro=sles15sp3 env=ONLY=55d testlist=sanityn

Change-Id: I42313e43eaea3d94007d534bf38efdeacf2ede43
Signed-off-by: Jian Yu <yujian@whamcloud.com>