lmb: correct size of the regions array

As in lmb_region, cnt < max and in the lmb library
use region[i] only with i in 0...cnt, this region array size
can be reduced by 1 element without overflow.

This patch allows to reduce the struct lmb size.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>