scsi: target: tcmu: Replace radix_tree with XArray

An attempt from Matthew Wilcox to replace radix-tree usage by XArray in
tcmu more than 1 year ago unfortunately got lost.

I rebased that work on latest tcmu and tested it.

Link: https://lore.kernel.org/r/20210224185335.13844-3-bostroesser@gmail.com


Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Bodo Stroesser <bostroesser@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>