ftp: fix CKSM command for files with white-space

The CKSM command allows a client to request the checksum of a
file.  It assumes that the path contains no whitespace and fails
any request where this isn't true.

This patch fixes this.

Target: master
Patch: http://rb.dcache.org/r/6853/
Acked-by: Gerd Behrmann
Request: 2.8
Request: 2.7
Request: 2.6
Merge pull request #486 from paulmillar/fix/2.8/ftp-fix-cksm

ftp: fix CKSM command for files with white-space