sd: Fix maximum I/O size for BLOCK_PC requests

sd: Fix maximum I/O size for BLOCK_PC requests

Commit bcdb247c6b6a ("sd: Limit transfer length") clamped the maximum
size of an I/O request to the MAXIMUM TRANSFER LENGTH field in the BLOCK
LIMITS VPD. This had the unfortunate effect of also limiting the maximum
size of non-filesystem requests sent to the device through sg/bsg.

Avoid using blk_queue_max_hw_sectors() and set the max_sectors queue
limit directly.

Also update the comment in blk_limits_max_hw_sectors() to clarify that
max_hw_sectors defines the limit for the I/O controller only.

Signed-off-by: Martin K. Petersen <[email protected]>
Reported-by: Brian King <[email protected]>
Tested-by: Brian King <[email protected]>
Cc: [email protected] # 3.17+
Signed-off-by: James Bottomley <[email protected]>