nfscl: Make nfscl_getlayout() acquire the correct pNFS layout

Without this patch, if a pNFS read layout has already been acquired
for a file, writes would be redirected to the Metadata Server (MDS),
because nfscl_getlayout() would not acquire a read/write layout for
the file. This happened because there was no "mode" argument to
nfscl_getlayout() to indicate whether reading or writing was being done.
Since doing I/O through the Metadata Server is not encouraged for some
pNFS servers, it is preferable to get a read/write layout for writes
instead of redirecting the write to the MDS.

This patch adds a access mode argument to nfscl_getlayout() and
nfsrpc_getlayout(), so that nfscl_getlayout() knows to acquire a read/write
layout for writing, even if a read layout has already been acquired.
This patch only affects NFSv4.1/4.2 client behaviour when pNFS ("pnfs" mount
option against a server that supports pNFS) is in use.

This problem was detected during a recent NFSv4 interoperability
testing event held by the IETF working group.

MFC after: 2 week