misc/tune2fs: fix setting fsuuid::fsu_len

misc/tune2fs: fix setting fsuuid::fsu_len

Minus does not mean equals.

Besides fixing an obvious bug, this avoids the following compiler
warning with clang -Wall:

tune2fs.c:3625:20: warning: expression result unused [-Wunused-value]
                        fsuuid->fsu_len - UUID_SIZE;
                        ~~~~~~~~~~~~~~~ ^ ~~~~~~~~~

Fixes: a83e199da0ca ("tune2fs: Add support for get/set UUID ioctls.")
Reviewed-by: Jeremy Bongio <bongiojp@gmail.com>
Reviewed-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>