audit: move the tree pruning to a dedicated thread

When file auditing is enabled, during a low memory situation, a memory
allocation with __GFP_FS can lead to pruning the inode cache.  Which can,
in turn lead to audit_tree_freeing_mark() being called.  This can call
audit_schedule_prune(), that tries to fork a pruning thread, and
waits until the thread is created.  But forking needs memory, and the
memory allocations there are done with __GFP_FS.

So we are waiting merrily for some __GFP_FS memory allocations to complete,
while holding some filesystem locks.  This can take a while ...

This patch creates a single thread for pruning the tree from
audit_add_tree_rule(), and thus avoids the deadlock that the on-demand
thread creation can cause.

Reported-by: Matt Wilson <msw@amazon.com>
Cc: Matt Wilson <msw@amazon.com>
Signed-off-by: Imre Palik <imrep@amazon.de>
Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <pmoore@redhat.com>

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