libblkid: use snprintf() instead of sprintf()

libblkid: use snprintf() instead of sprintf()

Replace sprintf() calls with snprintf() to ensure proper bounds
checking when formatting strings.

In encode.c, the check now validates snprintf() return value instead
of pre-checking buffer size, providing more robust error handling.

In probe.c, snprintf() is used with proper size calculation based on
remaining buffer space.

Signed-off-by: Karel Zak <kzak@redhat.com>