null_blk: Use strscpy() instead of strscpy_pad() in null_add_dev()

null_blk: Use strscpy() instead of strscpy_pad() in null_add_dev()

blk_mq_alloc_disk() already zero-initializes the destination buffer,
making strscpy() sufficient for safely copying the disk's name. The
additional NUL-padding performed by strscpy_pad() is unnecessary.

If the destination buffer has a fixed length, strscpy() automatically
determines its size using sizeof() when the argument is omitted. This
makes the explicit size argument unnecessary.

The source string is also NUL-terminated and meets the __must_be_cstr()
requirement of strscpy().

No functional changes intended.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20250410154727.883207-1-thorsten.blum@linux.dev
Signed-off-by: Jens Axboe <axboe@kernel.dk>