btrfs: Fix compile error when CONFIG_SECURITY is not set.
Fix the following compile error when CONFIG_SECURITY is not set:
error: 'struct security_mnt_opts' has no member named 'num_mnt_opts'
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>