Sanity check the flags passed to change_mnt_propagation().  Exactly
one flag should be set.  Return EINVAL otherwise.

Userspace can pass in arbitrary combinations of MS_* flags to mount().
do_change_type() is called if any of MS_SHARED, MS_PRIVATE, MS_SLAVE,
or MS_UNBINDABLE is set.  do_change_type() clears MS_REC and then
calls change_mnt_propagation() with the rest of the user-supplied
flags.  change_mnt_propagation() clearly assumes only one flag is set
but do_change_type() does not check that this is true.  For example,
mount() with flags MS_SHARED | MS_RDONLY does not actually make the
mount shared or read-only but does clear MNT_UNBINDABLE.
Signed-off-by: NValerie Aurora <vaurora@redhat.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>