isofs: fix setting of uid and gid to 0

isofs allows setting of default uid and gid of files but value 0 was used
to indicate that user did not specify any uid/gid mount option.  Since
this option also overrides uid/gid set in Rock Ridge extension, it makes
sense to allow forcing uid/gid 0.  Fix option processing to allow this.

Cc: <Hans-Joachim.Baader@cjt.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>