scsi: target: tcmu: Fix size in calls to tcmu_flush_dcache_range

1) If remaining ring space before the end of the ring is smaller then the
   next cmd to write, tcmu writes a padding entry which fills the remaining
   space at the end of the ring.

   Then tcmu calls tcmu_flush_dcache_range() with the size of struct
   tcmu_cmd_entry as data length to flush.  If the space filled by the
   padding was smaller then tcmu_cmd_entry, tcmu_flush_dcache_range() is
   called for an address range reaching behind the end of the vmalloc'ed
   ring.

   tcmu_flush_dcache_range() in a loop calls
   flush_dcache_page(virt_to_page(start)); for every page being part of the
   range. On x86 the line is optimized out by the compiler, as
   flush_dcache_page() is empty on x86.

   But I assume the above can cause trouble on other architectures that
   really have a flush_dcache_page().  For paddings only the header part of
   an entry is relevant due to alignment rules the header always fits in
   the remaining space, if pa...