nfs/localio: refactor iocb initialization

nfs/localio: refactor iocb initialization

The goal of this commit's various refactoring is to have LOCALIO's per
IO initialization occur in process context so that we don't get into a
situation where IO fails to be issued from workqueue (e.g. due to lack
of memory, etc). Better to have LOCALIO's iocb initialization fail
early.

There isn't immediate need but this commit makes it possible for
LOCALIO to fallback to NFS pagelist code in process context to allow
for immediate retry over RPC.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>