reiserfs: replace deprecated strncpy with scnprintf

strncpy() is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

Our goal here is to get @namebuf populated with @name's contents but
surrounded with quotes. There is some careful handling done to ensure we
properly truncate @name so that we have room for a literal quote as well
as a NUL-term. All this careful handling can be done with scnprintf
using the dynamic string width specifier %.*s which allows us to pass in
the max size for a source string. Doing this, we can put literal quotes
in our format specifier and ensure @name is truncated to fit inbetween
these quotes (-3 is from 2 quotes + 1 NUL-byte).

All in all, we get to remove a deprecated use of strncpy and clean up
this code nicely!

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://github.com/KSPP/linux/issues/90
Cc:  <linux-hardening@vger.kernel.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20240328-strncpy-fs-reiserfs-item_ops-c-v1-1-2dab6d22a996@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>