nfsd: check permissions when setting ACLs

Use set_posix_acl, which includes proper permission checks, instead of
calling ->set_acl directly.  Without this anyone may be able to grant
themselves permissions to a file by setting the ACL.

Lock the inode to make the new checks atomic with respect to set_acl.
(Also, nfsd was the only caller of set_acl not locking the inode, so I
suspect this may fix other races.)

This also simplifies the code, and ensures our ACLs are checked by
posix_acl_valid.

The permission checks and the inode locking were lost with commit
4ac7249e, which changed nfsd to use the set_acl inode operation directly
instead of going through xattr handlers.

Reported-by: David Sinquin <david@sinquin.eu>
[agreunba@redhat.com: use set_posix_acl]
Fixes: 4ac7249e
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
static int nfsaclsvc_encode_accessres(struct svc_rqst *rqstp, __be32 *p,
		struct nfsd3_accessres *resp)
{
	p = nfs2svc_encode_fattr(rqstp, p, &resp->fh, &resp->stat);
	*p++ = htonl(resp->access);
	return xdr_ressize_check(rqstp, p);
}

static int nfsaclsvc_encode_accessres(struct svc_rqst *rqstp, __be32 *p,
		struct nfsd3_accessres *resp)
{
	p = nfs2svc_encode_fattr(rqstp, p, &resp->fh, &resp->stat);
	*p++ = htonl(resp->access);
	return xdr_ressize_check(rqstp, p);
}
