cifs: skip tests that need POSIX support for nounix mounts

cifs: skip tests that need POSIX support for nounix mounts

CIFS/SMB protocol without POSIX extensions doesn't support operations
with symbolic links and advisory byte-range locks from the same process.
Add checks these features and use them in generic tests that require
such operations.

Signed-off-by: Pavel Shilovsky <pshilovsky@samba.org>
Reviewed-by: Steve French <smfrench@gmail.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>