dm crypt: replace open-coded kmemdup_nul

kzalloc() followed by strncpy() on an expected NUL-terminated string is
just kmemdup_nul(). Let's simplify this code (while also dropping a
deprecated strncpy() call [1]).

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://github.com/KSPP/linux/issues/90


Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>