nfscl: Add support for flexible file layout striping

Commit 72e57bc26417 added support for striping to the pNFS
server configuration. This patch adds support for striping
to the NFS client.

For striped flexible file layouts, an extra structure
must be malloc()d for each stripe, since the number
of stripe servers can vary from one mirror to another.
This new structure is called nfsffs and a single one
of these structures is in the nfsffm structure so that
the non-striped layouts can avoid the additional malloc()'s.

This patch only affects NFSv4.1/4.2 mounts that use the
"pnfs" mount option against servers that support the
flexible file layout.