b=1614

b=1614
r=phil

This bug fix was complicated by the fact that many code paths that
never had run now execute.  We changed intent error returns (already
committed), and error returns inside mds_open.

This includes a kernel fix but no version change, as I made it
backward compatible.

Fixes for open:
 - kernel passes FMODE_EXEC to lustre, which sends it to MDS
 - move all open/close code to mds_open.c, make almost everything static
 - arrange for allow/deny/get/put write access in mds
 - fix several problems with error handling in mds_dentry_open:
   - don't do a wrong mntput
   - do a dget for readability
 - add test to sanityN and sanity
 - simplify confusing permission checking in mds_open with helper