NFSv4.1: Fix exclusive create

When we use EXCLUSIVE4_1 mode, the server returns an attribute mask where
all the bits indicate which attributes were set, and where the verifier
was stored. In order to figure out which attribute we have to resend,
we need to clear out the attributes that are set in exclcreat_bitmask.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
[Anna: Fixed typo NFS4_CREATE_EXCLUSIVE4 -> NFS4_CREATE_EXCLUSIVE]
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>