nfsv41: consult backend 'filesystem' for read/write access

NFS implements truncate by setting file size to zero.
For this client should opbtain a write lock (open for write).
While permission checking done by PseudoFS, in some cases
we need more information than that. Let ask backend
filesystem for access rights.

The Chimera implementation reacts as following:

On a write check file location on a pool or on a tape.
This approach does not solves concurrent write issue,
but prevents existing files from corruptions.

Ticket: #7572
Acked-by: Karsten Schwank