[CIFS] Add support for new POSIX unlink

In the cleanup phase of the dbench test, we were noticing sharing
violation followed by failed directory removals when dbench
did not close the test files before the cleanup phase started.
Using the new POSIX unlink, which Samba has supported for a few
months, avoids this.

Signed-off-by: Steve French <sfrench@us.ibm.com>