NFS: Clean-up: use correct type when converting NFS blocks to local blocks

inode->i_blocks is a blkcnt_t these days, which can be a u64 or unsigned
long, depending on the setting of CONFIG_LSF.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>