ipv4: ip_tunnel: Replace strcpy use with strscpy

Use of strcpy is decpreated, replaces the use of strcpy with strscpy as
recommended.

strscpy was chosen as it requires a NUL terminated non-padded string,
which is the case here.

I am aware there is an explicit bounds check above the second instance,
however using strscpy protects against buffer overflows in any future
code, and there is no good reason I can see to not use it.

I have also replaced the scrscpy above that had 3 params with the
version using 2 params. These are functionally equivalent, but it is
cleaner to have both using 2 params.

Signed-off-by: Ruben Wauters <rubenru09@aol.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250501202935.46318-1-rubenru09@aol.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>