strncpy() is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces. Also replace an instance of strcpy() which is also
deprecated.

s_volume_ident is a NUL-terminated string which is evident from its
usage in udf_debug:
|	udf_debug("volIdent[] = '%s'\n", UDF_SB(sb)->s_volume_ident);

s_volume_ident should also be NUL-padded as it is copied out to
userspace:
|	if (copy_to_user((char __user *)arg,
|				UDF_SB(inode->i_sb)->s_volume_ident, 32))
|		return -EFAULT;

Considering the above, a suitable replacement is `strscpy_pad` [2] due
to the fact that it guarantees both NUL-termination and NUL-padding on
the destination buffer.

To simplify the code, let's use the new 2-argument version of
strscpy_pad() introduced in Commit e6584c39 ("string: Allow
2-argument strscpy()"). Also zero-allocate @outstr so we can safely use
a non-@ret length argument. This is just in case udf_dstrCS0toChar()
doesn't include the NUL-byte in its return length, we won't truncate
@outstr or write garbage bytes either.

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>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20240401-strncpy-fs-udf-super-c-v1-1-80cddab7a281@google.com>