mpr, mps: Fix an off-by-one bug in the BTDH_MAPPING ioctl
The device mapping table contains sc->max_devices entries, so only
indices in [0, sc->max_devices) are valid.
Differential Revision: https://reviews.freebsd.org/D27964
(cherry picked from commit adc0dcc352bb9f5a67a054d95c6959ea5aa26d91)