mempolicy: kill do_set_mempolicy()->down_write(&mm->mmap_sem)

mempolicy: kill do_set_mempolicy()->down_write(&mm->mmap_sem)

Remove down_write(&mm->mmap_sem) in do_set_mempolicy(). This logic
was never correct and it is no longer needed, see the previous patch.

Signed-off-by: Oleg Nesterov <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>