err.h: add (missing) unlikely() to IS_ERR_OR_NULL()

IS_ERR_VALUE() already contains it and so we need to add this only to
the !ptr check.  That will allow users of IS_ERR_OR_NULL(), to not add
this compiler flag.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>