Enable setting security contexts on rootfs inodes.

rootfs (ramfs) can support setting of security contexts
by userspace due to the vfs fallback behavior of calling
the security module to set the in-core inode state
for security.* attributes when the filesystem does not
provide an xattr handler.  No xattr handler required
as the inodes are pinned in memory and have no backing
store.

This is useful in allowing early userspace to label individual
files within a rootfs while still providing a policy-defined
default via genfs.

Change-Id: Iea1998cdeabcdb09036c5ca36b9d16d24bae4a2e
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Chirayu Desai <chirayudesai1@gmail.com>