NFSv4: Fix exclusive create attributes encoding

When using NFS4_CREATE_EXCLUSIVE4_1 mode, the client will overestimate the
amount of space that it needs for the attributes because it does so
before checking whether or not the server supports a given attribute.

Fix by checking the attribute mask earlier.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>