scsi: mesh: remove DISABLE_CLUSTERING

mesh has no limitations on crossing pages for segments.  Just make
the 65535 byte segment size limit explicit, even if it matches the
current block layer limit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>