bdi: use deferable timer for sync_supers task

sync_supers task currently wakes up periodically for superblock
writeback. This hurts power on battery driven devices. This patch
turns this housekeeping timer into a deferable timer so that it
does not fire when system is really idle.

CRs-Fixed: 353700
Change-Id: Idc7953b5d0580546808bc5832291ca570837ee7f
Signed-off-by: Yong Wang <yong.y.wang@intel.com>
Signed-off-by: Xia Wu <xia.wu@intel.com>
Signed-off-by: Krishna Vanka <kvanka@codeaurora.org>
Patch-mainline: v3.5
Git-commit: 19b52abe3c5d759661500a1dc810924369b2ad46
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
mm: prepare for removal of obsolete /proc/sys/vm/nr_pdflush_threads

Since per-BDI flusher threads were introduced in 2.6, the pdflush
mechanism is not used any more.  But the old interface exported through
/proc/sys/vm/nr_pdflush_threads still exists and is obviously useless.

For back-compatibility, printk warning information and return 2 to notify
the users that the interface is removed.

Signed-off-by: Wanpeng Li <liwp@linux.vnet.ibm.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>