Btrfs: fix printing of non NULL terminated string

The name buffer is not terminated by a '\0' character,
therefore it needs to be printed with %.*s and use the
length of the buffer.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>