smb: client: short-circuit negative lookups when parent dir is fully cached

smb: client: short-circuit negative lookups when parent dir is fully cached

When the parent directory has a valid and complete cached enumeration we
can assume that negative dentries are not present in the directory, thus
we can return without issuing a request.

This reduces traffic for common ENOENT when the directory entries are
cached.

Signed-off-by: Henrique Carvalho <henrique.carvalho@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>