checkpatch: struct seq_operations should normally be const

In the general use case struct seq_operations should be a const object.
Check for and warn where it is not.

Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>