scsi: ufs: core: Validate string descriptors

In the Linux kernel, the following vulnerability has been resolved:

scsi: ufs: core: Validate string descriptors

The string descriptor length includes a two-byte header while the UTF-16
payload starts after it. utf16s_to_utf8s() expects a count of UTF-16 code
units, not bytes. Passing the payload byte count can make it read beyond
the descriptor buffer.

Validate that the payload has an even byte count, pass a code-unit count to
the converter, and allocate sufficient UTF-8 output space.

The raw string buffer starts after the descriptor header but its size is
bLength. Copying bLength bytes from that pointer can read beyond the
response buffer.

Allocate a zeroed bLength-sized buffer and copy only the UTF-16
payload. This preserves the raw buffer size consumed by the RPMB device-ID
ABI while avoiding the overread.