Revert "SELinux: build fix for 4.1"

This reverts commit 43e1b4f5.

This patch is part of code which is already upstreamed in v4.4. Commits
5c73fceb (SELinux: Enable setting security contexts on rootfs inodes.),
12f348b9 (SELinux: rename SE_SBLABELSUPP to SBLABEL_MNT), and
b43e725d (SELinux: use a helper function to determine seclabel).
for reference.

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>