usb: gadget: f_midi: Replace strlcpy() with strscpy()

strlcpy() reads the entire source buffer first. This read may exceed
the destination size limit. This is both inefficient and can lead
to linear read overflows if a source string is not NUL-terminated[1].
Additionally, it returns the size of the source string, not the
resulting size of the destination string. In an effort to remove strlcpy()
completely[2], replace strlcpy() here with strscpy().

In the unlikely (impossible?) case where opts->id was larger than
PAGE_SIZE, this will now correctly report truncation errors.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy [1]
Link: https://github.com/KSPP/linux/issues/89 [2]
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Will McVicker <willmcvicker@google.com>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Peter Chen <peter.chen@nxp.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Zhang Qilong <zhangqilong3@huawei.com>
Cc: Linyu Yuan <quic_linyyuan@quicinc.com>
Cc: John Keeping <john@keeping.me.uk>
Cc: Azeem Shaikh <azeemshaikh38@gmail.com>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20231116191452.work.902-kees@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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