ocfs2: Fail ocfs2_get_block() immediately when a block needs allocation

ocfs2_get_block() does no allocation.  Hole filling for writes should
have happened farther up in the call chain.  We detect this case and
print an error, but we then continue with the function.  We should be
exiting immediately.

Signed-off-by: Wengang Wang <wen.gang.wang@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>