RDMA: Use __packed annotation instead of __attribute__ ((packed))

"__attribute__" set of macros has been standardized, have became more
potentially portable and consistent code back in v2.6.21 by commit
82ddcb04

 ("[PATCH] extend the set of "__attribute__" shortcut macros").
Moreover, nowadays checkpatch.pl warns about using __attribute__((packed))
instead of __packed.

This patch converts all the "__attribute__ ((packed))" annotations to
"__packed" within the RDMA subsystem.

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