USB: gadget: Replace deprecated strncpy() with strscpy()

USB: gadget: Replace deprecated strncpy() with strscpy()

strncpy() is deprecated for NUL-terminated destination buffers; use
strscpy() instead. Since kzalloc() already zeroes out the destination
buffer, the potential NUL-padding by strncpy() is unnecessary. strscpy()
copies only the required characters and guarantees NUL-termination.

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

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

No functional changes intended.

Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Kees Cook <kees@kernel.org>
Link: https://lore.kernel.org/r/20250320165647.34859-2-thorsten.blum@linux.dev
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>