NFS: Squelch compiler warning

Seen with -Wextra:

/home/cel/linux/fs/nfs/fscache.c: In function ‘__nfs_readpages_from_fscache’:
/home/cel/linux/fs/nfs/fscache.c:479: warning: comparison between signed and unsigned integer expressions

The comparison implicitly converts "int" to "unsigned", making it
safe.  But there's no need for the implicit type conversions here, and
the dfprintk() already uses a "%u" formatter for "npages."  Better to
reduce confusion.

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