ocfs2: Fix checking of return value of new_inode()

new_inode() does not return ERR_PTR() but NULL in case of failure. Correct
checking of the return value.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>