zram: avoid kunmap_atomic() of a NULL pointer

zram could kunmap_atomic() a NULL pointer in a rare situation: a zram
page becomes a full-zeroed page after a partial write io.  The current
code doesn't handle this case and performs kunmap_atomic() on a NULL
pointer, which panics the kernel.

This patch fixes this issue.

Signed-off-by: Weijie Yang <weijie.yang@samsung.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Weijie Yang <weijie.yang.kh@gmail.com>
Acked-by: Jerome Marchand <jmarchan@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
cpufreq: Add interface for gov change and upstream fixes

* Thanks @Dorimanx
* Interface can be used by drivers to change gov.

Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
msm: limiter: Add controls for per cpu governor

* Requires hooks in CPUFREQ driver.
* Works without limiter enabled.

Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>