test_sysclt: Test for registering a mount point

Test that target gets created by register_sysctl_mount_point and that no
additional target can be created "on top" of a permanently empty sysctl
table.

Create a mount point target (mnt) in the sysctl test driver; try to
create another on top of that (mnt_error). Output an error if
"mnt_error" is present when we run the sysctl selftests.

Signed-off-by: Joel Granados <j.granados@samsung.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>