sgi-xp: Use the bitmap API to allocate bitmaps

Use bitmap_zalloc()/bitmap_free() instead of hand-writing them.

It is less verbose and it improves the semantic.

While at it, remove a useless cast in a bitmap_empty() call.

Reviewed-by: Steve Wahl <steve.wahl@hpe.com>
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/ef49726d60f6a531428609f60a2398b6c3d9a26e.1656966181.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>