fs/namespace.c: fix the namespace_sem guard mess

fs/namespace.c: fix the namespace_sem guard mess

If anything, namespace_lock should be DEFINE_LOCK_GUARD_0, not DEFINE_GUARD.
That way we
	* do not need to feed it a bogus argument
	* do not get gcc trying to compare an address of static in
file variable with -4097 - and, if we are unlucky, trying to keep
it in a register, with spills and all such.

The same problems apply to grabbing namespace_sem shared.

Rename it to namespace_excl, add namespace_shared, convert the existing users:

    guard(namespace_lock, &namespace_sem) => guard(namespace_excl)()
    guard(rwsem_read, &namespace_sem) => guard(namespace_shared)()
    scoped_guard(namespace_lock, &namespace_sem) => scoped_guard(namespace_excl)
    scoped_guard(rwsem_read, &namespace_sem) => scoped_guard(namespace_shared)

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>