IB/ehca: Assure 4K alignment for firmware control blocks

Assure 4K alignment for firmware control blocks in 64K page mode,
because kzalloc()'s result address might not be 4K aligned if 64K
pages are enabled. Thus, we introduce wrappers called
ehca_{alloc,free}_fw_ctrlblock(), which use a slab cache for objects
with 4K length and 4K alignment in order to alloc/free firmware
control blocks in 64K page mode. In 4K page mode those wrappers just
are defines of get_zeroed_page() and free_page().

Signed-off-by: Hoang-Nam Nguyen <hnguyen@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>