MAX_LFS_FILESIZE should be a loff_t

fs/nfs/internal.h: In function ‘nfs_super_set_maxbytes’:
fs/nfs/internal.h:547:21: warning: comparison between signed and
    unsigned integer expressions [-Wsign-compare]

Seen with gcc (GCC) 4.6.3 20120306 (Red Hat 4.6.3-2).

Commit 42cb56ae made s_maxbytes a loff_t, thus the type of
MAX_LFS_FILESIZE should also be a loff_t.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>