staging/lustre: fix build error in ll_setxattr_common

With CONFIG_FS_POSIX_ACL off, we'll get
drivers/staging/lustre/lustre/llite/xattr.c: In function ‘ll_setxattr_common’:
drivers/staging/lustre/lustre/llite/xattr.c:187:7: error: ‘rce’ undeclared (first use in this function)
drivers/staging/lustre/lustre/llite/xattr.c:187:7: note: each undeclared identifier is reported only once for each function it appears in

It was introduced by 7fc1f831 (staging/lustre/llite: extended attribute cache)
that added new reference but didn't take care of CONFIG_FS_POSIX_ACL.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>