orangefs: make struct orangefs_file_vm_ops static

The struct orangefs_file_vm_ops is local to the source and does not need
to be in global scope, so make it static.

Cleans up sparse warning:
fs/orangefs/file.c:547:35: warning: symbol 'orangefs_file_vm_ops' was not
declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>