NFS: Rename nfs_commit_unstable_pages() to nfs_write_inode()

All nfs_write_inode() does is pass its arguments to
nfs_commit_unstable_pages().  Let's cut out the middle man and have
nfs_write_pages() do the work directly.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>