NFSv4.1: Fix a bad reference count issue in the pNFS commit code

filelayout_scan_commit_lists needs to bump the reference count on
the struct nfs_page just like nfs_scan_commit_list().

Reported-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>