udev-util: bound leading whitespace skip in udev_replace_whitespace

udev-util: bound leading whitespace skip in udev_replace_whitespace

The function documents that at most 'len' bytes are read from 'str',
but the leading whitespace skip used strspn(), which is bounded only
by a non-whitespace byte or a NUL. ata_id passes the space padded,
non-NUL-terminated ATA IDENTIFY fields, so an all-blank model reads
past the buffer. Use strnspn() to cap the skip to 'len'.