Since commit 4c395296 ("slab: Warn on duplicate cache names when
DEBUG_VM=y"), slab complains about duplicate cache names. Hence this
patch. The approach is as follows:
- Maintain an xarray with the slab size as index and a reference count
  and a kmem_cache pointer as contents. Use srpt-${slab_size} as kmem
  cache name.
- Use 512-byte alignment for all slabs instead of only for some of the
  slabs.
- Increment the reference count instead of calling kmem_cache_create().
- Decrement the reference count instead of calling kmem_cache_destroy().

Fixes: 5dabcd04 ("RDMA/srpt: Add support for immediate data")
Link: https://patch.msgid.link/r/20241009210048.4122518-1-bvanassche@acm.orgReported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Closes: https://lore.kernel.org/linux-block/xpe6bea7rakpyoyfvspvin2dsozjmjtjktpph7rep3h25tv7fb@ooz4cu5z6bq6/Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Tested-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>