err.h: add helper function to simplify pointer error checking

There are quite a few instances in the kernel of checks of pointers both
against NULL and against the errno range, handling both cases identically.
This additional helper function would simplify such code.

[akpm@linux-foundation.org: build fix]
Signed-off-by: Phil Carmody <ext-phil.2.carmody@nokia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>