ceph: wake up writer if vmtruncate work get blocked

To write data, the writer first acquires the i_mutex, then try getting
caps. The writer may sleep while holding the i_mutex. If the MDS revokes
Fb cap in this case, vmtruncate work can't do its job because i_mutex
is locked. We should wake up the writer and let it truncate the pages.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Reviewed-by: Sage Weil <sage@inktank.com>