CRED: Fix NFSD regression

Fix a regression in NFSD's permission checking introduced by the credentials
patches.  There are two parts to the problem, both in nfsd_setuser():

 (1) The return value of set_groups() is -ve if in error, not 0, and should be
     checked appropriately.  0 indicates success.

 (2) The UID to use for fs accesses is in new->fsuid, not new->uid (which is
     0).  This causes CAP_DAC_OVERRIDE to always be set, rather than being
     cleared if the UID is anything other than 0 after squashing.

Reported-by: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: James Morris <jmorris@namei.org>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>