Fix failure path in alloc_pid()

The failure path removes the allocated PIDs from the wrong namespace.
This could lead to us inadvertently reusing PIDs in the leaf namespace
and leaking PIDs in parent namespaces.

Fixes: 95846ecf ("pid: replace pid bitmap implementation with IDR API")
Cc: <stable@vger.kernel.org>
Signed-off-by: Matthew Wilcox <willy@infradead.org>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>