selinux: do not override context on context mounts

Ignore all selinux_inode_notifysecctx() calls on mounts with SBLABEL_MNT
flag unset. This is achived by returning -EOPNOTSUPP for this case in
selinux_inode_setsecurtity() (because that function should not be called
in such case anyway) and translating this error to 0 in
selinux_inode_notifysecctx().

This fixes behavior of kernfs-based filesystems when mounted with the
'context=' option. Before this patch, if a node's context had been
explicitly set to a non-default value and later the filesystem has been
remounted with the 'context=' option, then this node would show up as
having the manually-set context and not the mount-specified one.

Steps to reproduce:
    # mount -t cgroup2 cgroup2 /sys/fs/cgroup/unified
    # chcon unconfined_u:object_r:user_home_t:s0 /sys/fs/cgroup/unified/cgroup.stat
    # ls -lZ /sys/fs/cgroup/unified
    total 0
    -r--r--r--. 1 root root system_u:object_r:cgroup_t:s0        0 Dec 13 10:41 cgroup.controllers
    -rw-r--r--. 1 root root system_u:object_r:cgroup_t:s0        0 Dec 13 10:41 cgroup.max.depth
    -rw-r--r--. 1 root root system_u:object_r:cgroup_t:s0        0 Dec 13 10:41 cgroup.max.descendants
    -rw-r--r--. 1 root root system_u:object_r:cgroup_t:s0        0 Dec 13 10:41 cgroup.procs
    -r--r--r--. 1 root root unconfined_u:object_r:user_home_t:s0 0 Dec 13 10:41 cgroup.stat
    -rw-r--r--. 1 root root system_u:object_r:cgroup_t:s0        0 Dec 13 10:41 cgroup.subtree_control
    -rw-r--r--. 1 root root system_u:object_r:cgroup_t:s0        0 Dec 13 10:41 cgroup.threads
    # umount /sys/fs/cgroup/unified
    # mount -o context=system_u:object_r:tmpfs_t:s0 -t cgroup2 cgroup2 /sys/fs/cgroup/unified

Result before:
    # ls -lZ /sys/fs/cgroup/unified
    total 0
    -r--r--r--. 1 root root system_u:object_r:tmpfs_t:s0         0 Dec 13 10:41 cgroup.controllers
    -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0         0 Dec 13 10:41 cgroup.max.depth
    -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0         0 Dec 13 10:41 cgroup.max.descendants
    -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0         0 Dec 13 10:41 cgroup.procs
    -r--r--r--. 1 root root unconfined_u:object_r:user_home_t:s0 0 Dec 13 10:41 cgroup.stat
    -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0         0 Dec 13 10:41 cgroup.subtree_control
    -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0         0 Dec 13 10:41 cgroup.threads

Result after:
    # ls -lZ /sys/fs/cgroup/unified
    total 0
    -r--r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.controllers
    -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.max.depth
    -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.max.descendants
    -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.procs
    -r--r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.stat
    -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.subtree_control
    -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.threads

Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Reviewed-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>