NFSv4.2: encode_readdir - only ask for labels when doing readdirplus

Currently, if the server is doing NFSv4.2 and supports labeled NFS, then
our on-the-wire READDIR request ends up asking for the label information,
which is then ignored unless we're doing readdirplus.
This patch ensures that READDIR doesn't ask the server for label information
at all unless the readdir->bitmask contains the FATTR4_WORD2_SECURITY_LABEL
attribute, and the readdir->plus flag is set.

While we're at it, optimise away the 3rd bitmap field if it is zero.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>