avoid endless loops in lib/swiotlb.c

avoid endless loops in lib/swiotlb.c

Commit 681cc5cd3efbeafca6386114070e0bfb5012e249 ("iommu sg merging:
swiotlb: respect the segment boundary limits") introduced two
possibilities for entering an endless loop in lib/swiotlb.c:

 - if max_slots is zero (possible if mask is ~0UL)
 - if the number of slots requested fits into a swiotlb segment, but is
   too large for the part of a segment which remains after considering
   offset_slots

This fixes them

Signed-off-by: Jan Beulich <[email protected]>
Cc: FUJITA Tomonori <[email protected]>
Cc: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>