LU-28 fileset: add fileset mount support

LU-28 fileset: add fileset mount support

This patch enables client to mount subdirectory as fileset.

usage: mount -t lustre mgsname:/fsname/subdir /mount/point

* mdt lookup fileset fid and return to client during mount.
* `lfs fid2path` support for fileset.

Test-Parameters: envdefinitions=FILESET=/somedir testlist=sanity
Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Signed-off-by: Kit Westneat <kwestneat@ddn.com>
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: I947785de3b053907600d1090c80e8f57396e1e93
Reviewed-on: http://review.whamcloud.com/5007
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>