xfs: remove kmem_alloc_io()

Since commit 59bb4798 ("mm, sl[aou]b: guarantee natural alignment
for kmalloc(power-of-two)"), the core slab code now guarantees slab
alignment in all situations sufficient for IO purposes (i.e. minimum
of 512 byte alignment of >= 512 byte sized heap allocations) we no
longer need the workaround in the XFS code to provide this
guarantee.

Replace the use of kmem_alloc_io() with kmem_alloc() or
kmem_alloc_large() appropriately, and remove the kmem_alloc_io()
interface altogether.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>