NFS: rename nfs_fs_context pointer arg in a few functions

Split out from commit "NFS: Add fs_context support."

Rename cfg to ctx in nfs_init_server(), nfs_verify_authflavors(),
and nfs_request_mount().  No functional changes.

Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>