Handle __ll_get_inode failure in Client::ll_file_layout.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Fix C_Block_Sync and ll_write_block deadlock.

Former cond_wait behavior was not properly conditional
on an actual barrier wait state.

In addition, don't wait for ack -and- sync on unstable writes.
For now, choose sync, which makes any following COMMIT wait
for Ceph's commit point on each previous, uncommitted write.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Fix locking in ll_write_block.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>