ocfs2: Cleanup dirent size check

The check to see if a new dirent would fit in an old one is pretty ugly, and
it's done at least twice. Clean things up by putting this in it's own
easier-to-read function.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>
Reviewed-by: Joel Becker <joel.becker@oracle.com>