The num_free field of mthca_buddy has a type of array of unsigned int
while it was allocated as an array of pointers.  On 64-bit platforms
this allocates twice more than required.  Fix this by allocating the
correct size for the type.

This is the same bug just fixed in mlx4 by Eli Cohen <eli@mellanox.co.il>.
Signed-off-by: Roland Dreier <roland@purestorage.com>