ENOSYS means that a nonexistent system call was called.  We have a
bad habit of using it for things like invalid operations on
otherwise valid syscalls.  We should avoid this in new code.

Pervasive incorrect usage of ENOSYS came up at the kernel summit ABI
review discussion.  Let's see if checkpatch can help.

I'll submit a separate patch for include/uapi/asm-generic/errno.h.
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>