sparc: Fix -Wstringop-overflow warning

sparc: Fix -Wstringop-overflow warning

Greetings,

GCC 7 introduced the -Wstringop-overflow flag to detect buffer overflows
in calls to string handling functions [1][2]. Due to the way
``empty_zero_page'' is declared in arch/sparc/include/setup.h, this
causes a warning to trigger at compile time in the function mem_init(),
which is subsequently converted to an error. The ensuing patch fixes
this issue and aligns the declaration of empty_zero_page to that of
other architectures. Thank you.

Cheers,
Orlando.

[1] https://gcc.gnu.org/ml/gcc-patches/2016-10/msg02308.html
[2] https://gcc.gnu.org/gcc-7/changes.html

Signed-off-by: Orlando Arias <oarias@knights.ucf.edu>
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Signed-off-by: David S. Miller <davem@davemloft.net>