RDMA: Handle AH allocations by IB/core

Simplify drivers by ensuring lifetime of ib_ah object. The changes
in .create_ah() go hand in hand with relevant update in .destroy_ah().

We will use this opportunity and convert .destroy_ah() to don't fail, as
it was suggested a long time ago, because there is nothing to do in case
of failure during destroy.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>