IB/mad: Fix sparse warning about gfp_t use

Properly convert gfp_t & result to bool to fix:

    drivers/infiniband/core/sa_query.c:621:33: warning: incorrect type in initializer (different base types)
    drivers/infiniband/core/sa_query.c:621:33:    expected bool [unsigned] [usertype] preload
    drivers/infiniband/core/sa_query.c:621:33:    got restricted gfp_t

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