gru: fix potential use-after-free when purging GRU tlbs

Fix potential SGI GRU bug that could cause a use-after-free.  If one
thread in a task is flushing the GRU and another thread destroys the GRU
context, there is the potential to access a table after it has been freed.

Copy the gms pointer to a local variable before unlocking the gts table.
Note that no refcnt is needed for the gms - the reference is held
indirectly by the task's mm_struct.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>