checkpatch: fix use via symlink, make missing spelling file non-fatal

Commit 66b47b4a ("checkpatch: look for common misspellings") made it
difficult to use checkpatch via a symlink.

Fix that and make a missing spelling.txt file non-fatal.  Emit a warning
when the spelling.txt file can not be opened.

Reference: http://xkcd.com/1172/



Signed-off-by: Joe Perches <joe@perches.com>
Reported-by: Jani Nikula <jani.nikula@intel.com>
Tested-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>