NFSD: Fix a bug in the NFSv4 'supported attrs' mandatory attribute

The fact that the filesystem doesn't currently list any alternate
locations does _not_ imply that the fs_locations attribute should be
marked as "unsupported".

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>