LU-10467 ptlrpc: change LONG_UNLINK to PTLRPC_REQ_LONG_UNLINK

LU-10467 ptlrpc: change LONG_UNLINK to PTLRPC_REQ_LONG_UNLINK

The name "LONG_UNLINK" is vague and generic.  Change it to
  PTLRPC_REQ_LONG_UNLINK
to make it clear it is about requests taking a long time,
and of interest to PTLRPC.

Test-Parameters: trivial
Suggested-by:  Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I990657da3ec780c982a7ae31c21e4c8c9064be17
Reviewed-on: https://review.whamcloud.com/38405
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>