LU-2241 symlink: fix off-by-one error when reading symlinks

LU-2241 symlink: fix off-by-one error when reading symlinks

This fixes and off-by-one error when reading symlinks of inode size.
The null character is not accounted for when checking bufferlength
vs. inode data size.

Also add regression test to sanity.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: If4464cac60d57012311226113ff38b9c28926958
Reviewed-on: http://review.whamcloud.com/4415
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>