UBIFS: fix division by zero

If fanout is 3, we have division by zero in
'ubifs_read_superblock()':

divide error: 0000 [#1] PREEMPT SMP

Pid: 28744, comm: mount Not tainted (2.6.27-rc4-ubifs-2.6 #23)
EIP: 0060:[<f8f9e3ef>] EFLAGS: 00010202 CPU: 0
EIP is at ubifs_reported_space+0x2d/0x69 [ubifs]
EAX: 00000000 EBX: 00000000 ECX: 00000000 EDX: 00000000
ESI: 00000000 EDI: f0ae64b0 EBP: f1f9fcf4 ESP: f1f9fce0
 DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>