checkpatch: report any absolute references to kernel source files

Absolute references to kernel source files are generally only useful
locally to the originator of the patch.  Check for any such references and
report them.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>