ublk: fix narrowing warnings in UAPI header

When a C++ file compiled with -Wc++11-narrowing includes the UAPI header
linux/ublk_cmd.h, ublk_sqe_addr_to_auto_buf_reg()'s assignments of u64
values to u8, u16, and u32 fields result in compiler warnings. Add
explicit casts to the intended types to avoid these warnings. Drop the
unnecessary bitmasks.

Reported-by: Uday Shankar <ushankar@purestorage.com>
Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Fixes: 99c1e4eb ("ublk: register buffer to local io_uring with provided buf index via UBLK_F_AUTO_BUF_REG")
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250621162842.337452-1-csander@purestorage.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>