rbd: have rbd_parse_args() report found mon_addrs size

The argument parsing routine already computes the size of the
mon_addrs buffer it extracts from the "command."  Pass it to the
caller so it can use it to provide the length to rbd_get_client().

Signed-off-by: Alex Elder <elder@dreamhost.com>

diff --cc drivers/block/rbd.c
Simple merge