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