tests/9p: merge v9fs_tlink() and do_hardlink()

As with previous patches, unify those 2 functions into a single function
v9fs_tlink() by using a declarative function arguments approach.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <cb4d42203e1e4e6027df4924bbe4bdbc002f668b.1664917004.git.qemu_oss@crudebyte.com>