cifs: increase maximum buffer size in CIFSSMBQAllEAs

It's 4000 now, but there's no reason to limit it to that. We should be
able to handle a response up to CIFSMaxBufSize.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>