drivers: block: Mark functions as static in drbd_main.c

Mark functions _drbd_send_uuids(), fill_bitmap_rle_bits() and
init_submitter() as static in drbd/drbd_main.c because they are
not used outside this file.

This eliminates the following warnings in drbd/drbd_main.c:
drivers/block/drbd/drbd_main.c:826:5: warning: no previous prototype for ‘_drbd_send_uuids’ [-Wmissing-prototypes]
drivers/block/drbd/drbd_main.c:1070:5: warning: no previous prototype for ‘fill_bitmap_rle_bits’ [-Wmissing-prototypes]
drivers/block/drbd/drbd_main.c:2592:5: warning: no previous prototype for ‘init_submitter’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>