nfsd: fix problem with setting ACL on directories

nfsd: fix problem with setting ACL on directories

This is a regression of 3.12 stable commit
ba1816b40a87a3f3c4ca9cd54a593a10d87bd391 (nfsd: fix NFS regression).

If a non-inherited ACL is set on a directory, nfsd will try to set the Posix
default ACL to NULL.  This gets converted to "" by generic_setxattr().
As "" is not a valid posix acl attribute value, this results in an error.

So instead of setting the xattr to NULL, remove it.

Fixes: ba1816b40a ("nfsd: fix NFS regression")
Signed-off-by: NeilBrown <neilb@suse.com>
Cc: Sergio Gelato <Sergio.Gelato@astro.su.se>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>