LU-3613 llite: Add ioctl to get parent fids from link EA.

Added LL_IOC_GETPARENT to retrieve the <parent_fid>/name(s) of a given
entry, based on its link EA. This saves multiple calls to
path2fid/fid2path.

Added llapi_path2parent() and llapi_fd2parent() wrappers.

Added '--parents' option to lfs path2fid to call this function.

New sanity test 154f to validate the behavior.

Signed-off-by: Thomas Leibovici <thomas.leibovici@cea.fr>
Signed-off-by: Henri Doreau <henri.doreau@cea.fr>
Change-Id: I10f9316da605050be8fc709fe88d87abe86e22a4
Reviewed-on: http://review.whamcloud.com/7069
Tested-by: Jenkins
Reviewed-by: Ned Bass <bass6@llnl.gov>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: frank zago <fzago@cray.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>