Fix cache tracking for small fragsizes

Fix cache tracking for small fragsizes

The tracked amount of disk cache used by a client can be incorrect when
afs_fsfragsize is tuned to below 1023. Since we round up to the next
nearest afs_fsfragsize+1 then round down to the kb, if afs_fsfragsize+1
< 1kb, we can undershoot the cache usage.

This enforces afs_fsfragsize to always be at least 1023.

Reviewed-on: http://gerrit.openafs.org/338
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>