ocfs2: Provide convenience function for ino lookup

A couple paths which needed to just match a parent dir + name pair to an
inode number were a bit messy because they had to deal with
ocfs2_find_files_on_disk() which returns a larger number of values. Provide
a convenience function, ocfs2_lookup_ino_from_name() which internalizes all
the extra accounting.

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