scsi: hisi_sas: Add hisi_sas_debugfs_alloc() to centralise allocation

We extract the code of memory allocate and construct an new function for
it. We think it's convenient for subsequent optimization.

Link: https://lore.kernel.org/r/1567774537-20003-12-git-send-email-john.garry@huawei.com


Signed-off-by: Luo Jiaxing <luojiaxing@huawei.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>