scsi: elx: Use 'bitmap_zalloc()' when applicable

'sli4->ext[i].use_map' is a bitmap. Use 'bitmap_zalloc()' to simplify code,
improve the semantic and avoid some open-coded arithmetic in allocator
arguments.

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

Link: https://lore.kernel.org/r/2a0a83949fb896a0a236dcca94dfdc8486d489f5.1635104793.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>