LU-5252 hsm: Do not unconditionally delete the HSM event fifo

LU-5252 hsm: Do not unconditionally delete the HSM event fifo

The lustreapi HSM library will use a pipe if it already exists.
However the deregistration will unconditionally delete that pipe,
which is not correct.

An admin may want to create the pipe with certain rights/ownership,
which the llapi_hsm_register_event_fifo() doesn't allow. In that case,
llapi_hsm_unregister_event_fifo() should not delete it.

This is a back port to b2_5.

Lustre-change: http://review.whamcloud.com/10809
Lustre-commit: 7b7e6e2fb692eb3fa8d386addbc33763acf1a397

Signed-off-by: frank zago <fzago@cray.com>
Signed-off-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Change-Id: I66e44f21217228c5f425e5a9312d8e381d564c7f
Reviewed-on: http://review.whamcloud.com/11421
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>