fs/nfsd: Delete invalid assignment statements in nfsd4_decode_exchange_id

READ_BUF(8);
dummy = be32_to_cpup(p++);
dummy = be32_to_cpup(p++);
...
READ_BUF(4);
dummy = be32_to_cpup(p++);

Assigning value to "dummy" here, but that stored value
is overwritten before it can be used.
At the same time READ_BUF() will re-update the pointer p.

delete invalid assignment statements

Signed-off-by: nixiaoming <nixiaoming@huawei.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <trondmy@hammerspace.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>