NFS: Make nfs_cache_array.size a signed integer

Eliminate a number of implicit type casts in comparisons, and these
compiler warnings:

fs/nfs/dir.c: In function ‘nfs_readdir_clear_array’:
fs/nfs/dir.c:264:16: warning: comparison between signed and unsigned
		integer expressions [-Wsign-compare]
fs/nfs/dir.c: In function ‘nfs_readdir_search_for_cookie’:
fs/nfs/dir.c:352:16: warning: comparison between signed and unsigned
		integer expressions [-Wsign-compare]
fs/nfs/dir.c: In function ‘nfs_do_filldir’:
fs/nfs/dir.c:769:38: warning: comparison between signed and unsigned
		integer expressions [-Wsign-compare]
fs/nfs/dir.c:780:9: warning: comparison between signed and unsigned
		integer expressions [-Wsign-compare]
Signed-off-by: NChuck Lever <chuck.lever@oracle.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/dir.c

@@ -207,7 +207,7 @@ struct nfs_cache_array_entry {
 };
 
 struct nfs_cache_array {
-	unsigned int size;
+	int size;
 	int eof_index;
 	u64 last_cookie;
 	struct nfs_cache_array_entry array[0];