- mount option -o discard works
- 32K entry limit for the trimming array

- added discard=%u option, which spcifies the minimum of blocks
- default value: 2
- trim will only be called, if this minimum legth in fs blocks are trimmed
  -> when set so 1, even copying causes trimming on the ssd, which would
     make jfs really slow....
  -> discard=2 seems to be a good default

/TR 2012-07-24


TODO
- update Documentation/filesystems/jfs.txt
- there should also some documentation for the FITRIM ioctl()... what is the
  place for such a thing?
  -> Documentation/ioctl/fitrim.txt ?