sparc64: Properly range check DAX completion index

Each Oracle DAX CCB has a corresponding completion area, and the required
number of areas must fit within a previously allocated array of completion
areas beginning at the requested index.  Since the completion area index
is specified by a file offset, a user can pass arbitrary values, including
negative numbers. So the index must be thoroughly range checked to prevent
access to addresses outside the bounds of the allocated completion
area array.  The index cannot be negative, and it cannot exceed the
total array size, less the number of CCBs requested. The old code did
not check for negative values and was off by one on the upper bound.

Signed-off-by: Rob Gardner <rob.gardner@oracle.com>
Signed-off-by: Jonathan Helman <jonathan.helman@oracle.com>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>