[GFS2] Export file_read_actor

Export file_read_actor so that it can be used from modules since
functions which take this function as an argument are already
exported.
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

mm/filemap.c

@@ -981,6 +981,7 @@ int file_read_actor(read_descriptor_t *desc, struct page *page,
 	desc->arg.buf += size;
 	return size;
 }
+EXPORT_SYMBOL(file_read_actor);
 
 /*
  * This is the "read()" routine for all filesystems