iscsi: Advertise realistic limits to block layer

iscsi: Advertise realistic limits to block layer

The function sector_limits_lun2qemu() returns a value in units of
the block layer's 512-byte sector, and can be as large as
0x40000000, which is much larger than the block layer's inherent
limit of BDRV_REQUEST_MAX_SECTORS.  The block layer already
handles '0' as a synonym to the inherent limit, and it is nicer
to return this value than it is to calculate an arbitrary
maximum, for two reasons: we want to ensure that the block layer
continues to special-case '0' as 'no limit beyond the inherent
limits'; and we want to be able to someday expand the block
layer to allow 64-bit limits, where auditing for uses of
BDRV_REQUEST_MAX_SECTORS will help us make sure we aren't
artificially constraining iscsi to old block layer limits.

Signed-off-by: Eric Blake <[email protected]>
Reviewed-by: Kevin Wolf <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>