drivers/misc: sram-exec: have the callers of set_memory_*() check the return value

Right now several architectures allow their set_memory_*() family of
functions to fail, but callers may not be checking the return values.
If set_memory_*() returns with an error, call-site assumptions may be
infact wrong to assume that it would either succeed or not succeed at
all. Ideally, the failure of set_memory_*() should be passed up the call
stack, and callers should examine the failure and deal with it.

Need to fix the callers and add the __must_check attribute. They also
may not provide any level of atomicity, in the sense that the memory
protections may be left incomplete on failure. This issue likely has a
few steps on effects architectures:
  1) Have all callers of set_memory_*() helpers check the return value.
  2) Add __must_check to all set_memory_*() helpers so that new uses do
     not ignore the return value.
  3) Add atomicity to the calls so that the memory protections aren't
     left in a partial state.

This series is part of step 1. Make sram driver check the return value
of set_memory_*().

Signed-off-by: Tianlin Li <tli@digitalocean.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20191217194528.16461-1-tli@digitalocean.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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