Export blk_fill_rwbs()

Exported so it can be used by bcache's tracepoints

Signed-off-by: Kent Overstreet <koverstreet@google.com>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: Ingo Molnar <mingo@redhat.com>