[CIFS] Respect umask when using POSIX mkdir

When making a directory with POSIX mkdir calls, cifs_mkdir does not
respect the umask.  This patch causes the new POSIX mkdir to create with
the right mode

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>