scsi: tcmu: Simplify tcmu_update_uio_info()

Use 'kasprintf()' instead of:
   - snprintf(NULL, 0...
   - kmalloc(...
   - snprintf(...

This is less verbose and saves 7 bytes (i.e. the space for '/(null)') if
'udev->dev_config' is NULL.

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