SUNRPC: remove non-exclusive pipe creation from RPC pipefs

This patch-set was created in context of clone of git branch:
git://git.linux-nfs.org/projects/trondmy/nfs-2.6.git

.

v2:
1) Rebased of current repo state (i.e. all commits were pulled before apply)

I feel it is ready for inclusion if no objections will appear.

SUNRPC pipefs non-exclusive pipe creation code looks obsolete. IOW, as I see
it, all pipes are creating with unique full path and only once.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>