RDMA/rxe: Use 'bitmap_zalloc()' when applicable

'index.table' is a bitmap. So use 'bitmap_zalloc()' to simplify code,
improve the semantic and avoid some open-coded arithmetic in allocator
arguments.

Using 'bitmap_zalloc()' also allows the removal of a now useless
'bitmap_zero()'.

Also change the corresponding 'kfree()' into 'bitmap_free()' to keep
consistency.

Link: https://lore.kernel.org/r/4a3e11d45865678d570333d1962820eb13168848.1635093628.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>