Wip inline

This patch implements inline data support for Ceph.

Against v5:
Remove two unnecessary lines of codes according to Zheng's comments

Against v4:
Forbid old mds without inline support to join inline ever enabled mds cluster
Kick out old mds without inline support when inline enabled
Return EPERM to old client without inline support when it is trying to open inlined file    
Use version_t instead of __u64 for inline version

Against v3:
Add the inline switch and make inline size tunable
Improve the back compatibility of mds from (9, 9) to (9, 6), provided the inlinefeature is disabled until all mds upgraded to support inline 

Against v2:
Streamline the inline data migration with the subsequent read/write

Against v1:
With simplified process under multiple-writer case,
referred to
http://pad.ceph.com/p/mds-inline-data,
http://www.spinics.net/lists/ceph-devel/msg16018.html