IB/core: convert to idr_alloc()

Convert to the much saner new idr interface.

v2: Mike triggered WARN_ON() in idr_preload() because send_mad(),
    which may be used from non-process context, was calling
    idr_preload() unconditionally.  Preload iff @gfp_mask has
    __GFP_WAIT.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Sean Hefty <sean.hefty@intel.com>
Reported-by: "Marciniszyn, Mike" <mike.marciniszyn@intel.com>
Cc: Roland Dreier <roland@kernel.org>
Cc: Sean Hefty <sean.hefty@intel.com>
Cc: Hal Rosenstock <hal.rosenstock@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>