nfsd4: fix READ permission checking

In the case we already have a struct file (derived from a stateid), we
still need to do permission-checking; otherwise an unauthorized user
could gain access to a file by sniffing or guessing somebody else's
stateid.

Cc: stable@vger.kernel.org
Fixes: dc97618d

 "nfsd4: separate splice and readv cases"
Signed-off-by: J. Bruce Fields <bfields@redhat.com>