saner handling of temporary namespaces

mount_subtree() creates (and soon destroys) a temporary namespace,
so that automounts could function normally.  These beasts should
never become anyone's current namespaces; they don't, but it would
be better to make prevention of that more straightforward.  And
since they don't become anyone's current namespace, we don't need
to bother with reserving procfs inums for those.

Teach alloc_mnt_ns() to skip inum allocation if told so, adjust
put_mnt_ns() accordingly, make mount_subtree() use temporary
(anon) namespace.  is_anon_ns() checks if a namespace is such.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>