xfs: convert buf_cancel_table allocation to kmalloc_array

Source kernel commit: 910bbdf2f4d7df46781bc9b723048f5ebed3d0d7

While we're messing around with how recovery allocates and frees the
buffer cancellation table, convert the allocation to use kmalloc_array
instead of the old kmem_alloc APIs, and make it handle a null return,
even though that's not likely.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>