string: drop __must_check from strscpy()

string: drop __must_check from strscpy()

This was done as part of commit 08a77676f9c5 upstream, from which
the following description is taken:

> strlcpy() is worse than strlcpy() because it unconditionally runs
> strlen() on the source string, and the only reason we switched to
> strlcpy() here was because it was lacking __must_check, which doesn't
> reflect any material differences between the two function.  It's just
> that someone added __must_check to strscpy() and not to strlcpy().
>
> These basic string copy operations are used in variety of ways, and
> one of not-so-uncommon use cases is safely handling truncated copies,
> where the caller naturally doesn't care about the return value.  The
> __must_check doesn't match the actual use cases and forces users to
> opt for inferior variants which lack __must_check by happenstance or
> spread ugly (void) casts.

Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>