scsi: ilog2: create truly constant version for sparse

Sparse emits errors about ilog2() in array indices because of the use of
__ilog2_32() and __ilog2_64(), rightly so
(https://www.spinics.net/lists/linux-sparse/msg03471.html

).

Create a const_ilog2() variant that works with sparse for this scenario.

(Note: checkpatch.pl complains about missing parentheses, but that
appears to be a false positive. I can get rid of the warning simply by
inserting whitespace, making checkpatch "see" the whole macro).

Signed-off-by: Martin Wilck <mwilck@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>