smb: server: Fix extension string in ksmbd_extract_shortname()

In ksmbd_extract_shortname(), strscpy() is incorrectly called with the
length of the source string (excluding the NUL terminator) rather than
the size of the destination buffer. This results in "__" being copied
to 'extension' rather than "___" (two underscores instead of three).

Use the destination buffer size instead to ensure that the string "___"
(three underscores) is copied correctly.

Cc: stable@vger.kernel.org
Fixes: e2f34481 ("cifsd: add server-side procedures for SMB3")
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>