nfsd4: READ, READDIR, etc., are idempotent

OP_MODIFIES_SOMETHING flags operations that we should be careful not to
initiate without being sure we have the buffer space to encode a reply.

None of these ops fall into that category.

We could probably remove a few more, but this isn't a very important
problem at least for ops whose reply size is easy to estimate.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>