xfrm: Avoid clang fortify warning in copy_to_user_tmpl()

After a couple recent changes in LLVM, there is a warning (or error with
CONFIG_WERROR=y or W=e) from the compile time fortify source routines,
specifically the memset() in copy_to_user_tmpl().

  In file included from net/xfrm/xfrm_user.c:14:
  ...
  include/linux/fortify-string.h:438:4: error: call to '__write_overflow_field' declared with 'warning' attribute: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror,-Wattribute-warning]
    438 |                         __write_overflow_field(p_size_field, size);
        |                         ^
  1 error generated.

While ->xfrm_nr has been validated against XFRM_MAX_DEPTH when its value
is first assigned in copy_templates() by calling validate_tmpl() first
(so there should not be any issue in practice), LLVM/clang cannot really
deduce that across the boundaries of these functions. Without that
knowledge, it cannot assume that the loop stops before i is greater than
XFRM_MAX_DEPTH, which would indeed result a stack buffer overflow in the
memset().

To make the bounds of ->xfrm_nr clear to the compiler and add additional
defense in case copy_to_user_tmpl() is ever used in a path where
->xfrm_nr has not been properly validated against XFRM_MAX_DEPTH first,
add an explicit bound check and early return, which clears up the
warning.

Cc: stable@vger.kernel.org
Link: https://github.com/ClangBuiltLinux/linux/issues/1985
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>