udf: move calculating of nr_groups into helper function

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: Ben Fennema <bfennema@falcon.csc.calpoly.edu>
Acked-by: Jan Kara <jack@suse.cz>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>