mlx4_core: Fix buddy->num_free allocation size

The num_free field of mlx4_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.

Signed-off-by: Eli Cohen <eli@mellanox.co.il>

[ Convert to kcalloc().  - Roland ]

Signed-off-by: Roland Dreier <roland@purestorage.com>