Fix cpuset sched_relax_domain_level control file

Due to a merge conflict, the sched_relax_domain_level control file was marked
as being handled by cpuset_read/write_u64, but the code to handle it was
actually in cpuset_common_file_read/write.

Since the value being written/read is in fact a signed integer, it should be
treated as such; this patch adds cpuset_read/write_s64 functions, and uses
them to handle the sched_relax_domain_level file.

With this patch, the sched_relax_domain_level can be read and written, and the
correct contents seen/updated.

Signed-off-by: Paul Menage <menage@google.com>
Cc: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Cc: Paul Jackson <pj@sgi.com>
Cc: Ingo Molnar <mingo@elte.hu>
Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>