Merge st_ioctl_common() into st_ioctl() and streamline the invocation of
the common ioctl helpers.

Link: https://lore.kernel.org/r/20210724072033.1284840-8-hch@lst.deSigned-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>