mountmgr: Return STATUS_BUFFER_OVERFLOW if an insufficient buffer is passed to IOCTL_MOUNTMGR_ENUMERATE_CREDENTIALS.

STATUS_MORE_ENTRIES is used for directory enumeration APIs, and signals that a
continuation will be returned on the next call.
Signed-off-by: Zebediah Figura <zfigura@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>