pnfs: clean up filelayout_alloc_commit_info

Remove unneeded else statement and clean up how commit info
dataserver buckets are replaced.

Suggested-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Weston Andros Adamson <dros@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>