-musage=>0 means always reporting relocation

[Russell Coker <russell@coker.com.au>]

Below is the output of running a balance a few times on a 120G SSD.

It seems that whenever I set the metadata usage to be greater than 0 it will 
report relocating something, regardless of whether that's possible.

root@server:~# btrfs fi balance start -dusage=0 -musage=0 /
Done, had to relocate 0 out of 108 chunks
root@server:~# btrfs fi balance start -dusage=0 -musage=0 /
Done, had to relocate 0 out of 108 chunks
root@server:~# btrfs fi balance start -dusage=0 -musage=10 /
Done, had to relocate 1 out of 108 chunks
root@server:~# btrfs fi balance start -dusage=0 -musage=10 /
Done, had to relocate 1 out of 108 chunks
root@server:~# btrfs fi balance start -dusage=0 -musage=10 /
Done, had to relocate 1 out of 108 chunks
root@server:~# btrfs fi balance start -dusage=0 -musage=10 /
Done, had to relocate 1 out of 108 chunks
root@server:~# btrfs fi balance start -dusage=0 -musage=1 /
Done, had to relocate 1 out of 108 chunks
root@server:~# btrfs fi balance start -dusage=0 -musage=0 /
Done, had to relocate 0 out of 108 chunks
root@server:~# btrfs fi balance start -dusage=1 -musage=0 /
Done, had to relocate 0 out of 108 chunks
root@server:~# btrfs fi balance start -dusage=10 -musage=0 /
Done, had to relocate 0 out of 108 chunks

During each run of -musage=N I'd get a couple of kernel lines like the 
following.  It was apparently doing something, but whatever that was probably 
wasn't useful.

[1394107.479342] BTRFS info (device dm-0): relocating block group 766120755200 
flags 34
[1394107.589661] BTRFS info (device dm-0): found 2 extents

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