hfsplus: refactor copy_name to not use strncpy

strncpy() is deprecated with NUL-terminated destination strings [1].

The copy_name() method does a lot of manual buffer manipulation to
eventually arrive with its desired string. If we don't know the
namespace this attr has or belongs to we want to prepend "osx." to our
final string. Following this, we're copying xattr_name and doing a
bizarre manual NUL-byte assignment with a memset where n=1.

Really, we can use some more obvious string APIs to acomplish this,
improving readability and security. Following the same control flow as
before: if we don't know the namespace let's use scnprintf() to form our
prefix + xattr_name pairing (while NUL-terminating too!). Otherwise, use
strscpy() to return the number of bytes copied into our buffer.
Additionally, for non-empty strings, include the NUL-byte in the length
-- matching the behavior of the previous implementation.

Note that strscpy() _can_ return -E2BIG but this is already handled by
all callsites:

In both hfsplus_listxattr_finder_info() and hfsplus_listxattr(), ret is
already type ssize_t so we can change the return type of copy_name() to
match (understanding that scnprintf()'s return type is different yet
fully representable by ssize_t). Furthermore, listxattr() in fs/xattr.c
is well-equipped to handle a potential -E2BIG return result from
vfs_listxattr():
|	ssize_t error;
...
|	error = vfs_listxattr(d, klist, size);
|	if (error > 0) {
|		if (size && copy_to_user(list, klist, error))
|			error = -EFAULT;
|	} else if (error == -ERANGE && size >= XATTR_LIST_MAX) {
|		/* The file system tried to returned a list bigger
|			than XATTR_LIST_MAX bytes. Not possible. */
|		error = -E2BIG;
|	}
... the error can potentially already be -E2BIG, skipping this else-if
and ending up at the same state as other errors.

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
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/20240401-strncpy-fs-hfsplus-xattr-c-v2-1-6e089999355e@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>