ipc: conserve sequence numbers in extended IPCMNI mode

The mixing in of a sequence number into the IPC IDs is probably to avoid
ID reuse in userspace as much as possible.  With extended IPCMNI mode, the
number of usable sequence numbers is greatly reduced leading to higher
chance of ID reuse.

To address this issue, we need to conserve the sequence number space as
much as possible.  Right now, the sequence number is incremented for every
new ID created.  In reality, we only need to increment the sequence number
when one or more IDs have been removed previously to make sure that those
IDs will not be reused when a new one is built.  This is being done only
in the new extended IPCMNI mode.

Link: http://lkml.kernel.org/r/1536352137-12003-5-git-send-email-longman@redhat.com
Signed-off-by: Waiman Long <longman@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kees Cook <keescook@chromium.org>
Cc: Luis R. Rodriguez <mcgrof@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

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