nbd: fix max_discard/max_transfer_length

nbd_co_discard calls nbd_client_session_co_discard which uses uint32_t
as the length in bytes of the data to discard due to the following
definition:

struct nbd_request {
    uint32_t magic;
    uint32_t type;
    uint64_t handle;
    uint64_t from;
    uint32_t len; <-- the length of data to be discarded, in bytes
} QEMU_PACKED;

Thus we should limit bl_max_discard to UINT32_MAX >> BDRV_SECTOR_BITS to
avoid overflow.

NBD read/write code uses the same structure for transfers. Fix
max_transfer_length accordingly.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Peter Lieven <pl@kamp.de>
CC: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
