scsi: bump up SD_MAX_DISKS

scsi: bump up SD_MAX_DISKS

SD_MAX_DISKS is arbitrarily limited to the number of scsi disks in the
namespace constructed of "sd" followed by one to three of the letters
a-z, or 18278 disks. There is no need for this limit, since appending
a fourth letter works perfectly fine. This simple patch just bumps the
number up to allow up to four letters after "sd". It might be best to
simply remove the test against SD_MAX_DISKS, but this is the patch that
has been unit tested.

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>